Ask a reliable source when you need to determine whether a particular kosher-supervision body is reliable. You do not need to do any further research.

Supervision is needed during the manufacturing of certain foods to certify they are kosher. These products are usually marked with a supervision mark (“hechsher”) of the certifying body.
 

You may trust that the food a shomer Shabbat Jew serves is kosher without your needing to check it out.
However, if a shomer Shabbat host serves non-kosher food or food without reliable supervision on foods that need supervision, you may not eat it.

You may not use vegetable oil that has been processed in non-kosher equipment.
Reason The main problem can arise from deodorization of the oil, which is done in hot vessels.

Five factors affect cooking separate pans of dairy and meat in an oven at the same time (these are all b'di'avad cases):

• Covered 

• Outside Clean and Dry

• Food Non-Liquid (“Solid”) 

• The beginning of the cooking,
• The end of the cooking, OR
• Both beginning and end of cooking.

• Pans Touching 

• Spicy/Charif

 

Gender/Kashrut Status Transfer
Foods and kitchenware (pots, pans, dishes, utensils, and containers) can absorb taste from each other and so adopt a new gender or kosher status. They can change from:

• Kosher to non-kosher,
• Kosher pareve (neutral) to kosher dairy or kosher meat, or
• Kosher Passover to kosher (or non-kosher) non-Passover.

 
Note You can sometimes change a utensil/container to kosher-pareve (see Kashering, below), but you cannot change a

• Gendered food to neutral-pareve, or
• Non-kosher food to kosher.

 
Taste Absorption
Taste gets absorbed in three ways: Heat, pressure, and soaking. 
 
Heat
To absorb taste, and therefore gender or kashrut status, through heat, a food or utensil must be heated to 120° F or more while:

• Steamed with a halachically “liquid” food, or
• In wet physical contact with the food or utensil.

Examples

• Two hot pans, which are clean on their outsides, only transfer taste from one to the other if they are wet on the outside and are touching each other.
• A hot utensil placed onto a counter only transfers gender to the countertop if there is liquid or food at the point of contact.

Note  All liquids plays a major role in facilitating taste transfer.
NOTE Taste, gender, or non-kosher status do not travel upstream into the utensil that food is being poured from. Even if you pour hot liquid (pareve or of one gender) from a pot onto a non-kosher or opposite gender food, the genders are not transferred back through the stream of liquid to the pot, even if any or all of the elements are more than 120 degrees.
Situation You pour hot liquid from some pareve vegetables into a non-kosher sink that had hot in it within 24 hours. There are dishes or utensils in the sink.
Status The dishes do not change gender unless the hot liquid fills up from the sink onto them. If so, the dishes or utensils become non-kosher. But no gender change occurs through the stream of liquid back to the pot of vegetables.
 
Note If the non-kosher sink had not had anything hot (120 degrees or above) in it for at least 24 hours, no change of gender or kosher status happens at all.
 
Note On Passover, gender and chametz status DO get transferred through a stream of hot liquid.
 
Pressure
To absorb taste, and therefore gender or kashrut status, through pressure or short-term soaking, one of the items must be spicy/charif.
 
Soaking
To absorb taste, and therefore gender or kashrut status, through long-term soaking, the food must soak for specific amounts of time.
 
Note If the food or utensil is not hot (120° F or more), is not spicy/charif, and is not soaking for a long time, there is no gender or kashrut-status transfer.
Examples
You may use a non-kosher utensil for any cold food of the opposite gender, so you may:

• Eat cold (kosher) cereal out of a meat or non-kosher bowl, or
• Use a meat or non-kosher spoon to eat kosher ice cream.

Note Even though these are permissible, they may not be done regularly but only on an ad hoc basis.
 
Food and Kitchenware: Which Influences What
Hot or Spicy/Charif Foods
With hot (more than 120° F) or spicy/charif foods:
Foods and utensils/containers transfer taste to each other.
 
Cold or Non-Spicy Foods that Soak
With cold (less than 120° F) or non-spicy/charif foods that soak:

• Foods do not transfer taste to utensils/containers;
• Utensils/containers do NOT transfer taste to foods.

NOTE No substances (not salt, or any food...) absorb gender from the open air.
 
 
The 24-Hour Rule: Eino ben Yomo
Torah Law: Reverts to Kosher-Pareve
By Torah law, a utensil/container always reverts to kosher-pareve after 24 hours (since the taste of any absorbed food becomes ruined with time). 
 
Rabbinic Law: Must Be Kashered
However, by rabbinic law, the utensil/container must be kashered before using.
 
NOTE Even by Torah law, a hot or spicy/charif food can revive the milk-meat or non-kosher status of another utensil/container (see below) even after 24 hours.
 
Accidentally or Intentionally
Food Hot and Accidentally Placed; Utensil Not Hot for 24 Hours
Kosher food hotter than 120° F (49° C) remains kosher if accidentally placed into a non-kosher, clean utensil that has not been heated to 120° F or more for at least 24 hours.
REASON After 24 hours, b'di'avad, the utensil has reverted to being kosher-pareve.
NOTE If the utensil had been “used” (heated to 120° F or more) within the preceding 24 hours, the hot food that accidentally entered the utensil would be non-kosher. Ask a rabbi for possible exceptions.
 
Food Hot and Intentionally Placed
If the hot food had been put into the utensil intentionally, the food would not be kosher.
REASON Chazal made a rule (takana) that if you intentionally place food of one gender into a utensil of the opposite gender and heat it to 120° F or more, the food is not kosher.

Ba'alei teshuva often have problems with issues of kashrut in their parents' homes. Pots, dishes, and utensils might not be kosher or toveled. Consult a rabbi. Questions of bishul akum (cooking that was done by a non-Jew) might apply to non-shomer Shabbat parents, but the custom is to be lenient.

If the parent's kitchen is known to be non-kosher, food must be prepared with care (see How To Use a Non-Kosher Kitchen). If the parents do not lie to their children, they may be trusted as to the source of food and its kosher status.

Since we may not eat from dishes or utensils that have not been toveled (immersed in a mikva), you may  want to consider toveling your parents' dishes or utensils, or using disposable goods. In such cases, it is OK to use china that has not been toveled.

All beer in the USA is kosher and does not need supervision. Many beers in other countries are also kosher even without supervision, but the status should be checked with local kashrut authorities.
Situation Some beers have lactose added.
Status If the lactose is less than 1/60^th of the total volume, the beer is not considered to be dairy.

Bugs are not kosher and may not be eaten for two main reasons:

• First, the Torah forbids the “sheretz ha'shoreitz al ha'aretz” (“creeping thing that creeps on the ground”).
• Second, if people consider bugs disgusting to eat, the bugs are forbidden.

               Note Eating a bug causes more infractions of halacha than eating pork!
Three basic principles govern bugs/insects in food:

1. Partial-Bug Nullification
   A whole bug or insect cannot be nullified, even when it makes up less than 1/60th of the volume of the food in which it is found.
   
   Note Any bug that is missing even a tiny part of its body may be nullified if less than 1/60th of the volume of the food in which it is found, but only if:

   • You cannot remove the bug, and
   • You cannot see it. 

   Note If you can remove the bug, you must remove it.
2. The Three-Bug Rule
   Any time you find three bugs in food, you must assume there are more bugs to be found and you must therefore check every piece of that food before eating any of it.
3. The Still-Kosher Food-and-Pot Rule
   Even if the bugs were cooked in the food, as long as you later remove all the bugs, the food and the utensil in which they were cooked remain kosher.
   
   Exception If bugs make up 50\% or more of the total volume, the food is not kosher.
   What To DoIn this case, you should consult a rabbi about whether the utensil is kosher.

Processed Food without Hechsher: Is It Kosher?
 
If a processed food does not have supervision/hashgacha, here are some issues to consider:

• Ingredients;
• Utensils/processing equipment;
• Bishul akum/“prestigious” foods that require Jewish involvement in the cooking;
• Heating system (recirculated steam?);
• Heter for milk without being supervised - which conditions and countries can be relied on;
• Non-food ingredients (lubricants, preservatives, emulsifiers...);
• Reliability of the producer;
• Is the non-kosher ingredient batel/nullified?
  • ownership (Is the food's producer or owner Jewish?)
  • intended consumer (Is the food being produced specifically for Jews, or is it for the public and Jews are some of the customers)?
  • Was the non-kosher substance added intentionally?
  • Does the non-kosher substance have flavor?
  • Was the non-kosher substance added for flavor?

A hechsher/kosher supervision is not needed on:

• Beer made in the US (and sometimes in other countries).
• Nuts (dry roasted) without additives.
• Olives--assumed to be kosher unless mixed with ingredients that may be non-kosher, such as:
  • Vinegar (sometimes made from grapes).
  • Non-kosher chemical preservatives (in commercially sold olives).
  Note In open markets in which olives are sold in bulk, you may eat olives after checking the ingredients.
• Olive oil (extra virgin).
• Pure fruit juice NOT made from concentrate (such as orange or pineapple juice) does not normally require a hechsher (except for grape juice, which always requires a hechsher!).
  Note Juices from concentrate might have kashrut problems due to the vats in which they are cooked or pasteurized. If you can verify how the juice was processed and that there are no kashrut problems, you may use the juice without a hechsher. There may also be problems with juice made from fruit or vegetables which were grown in Eretz Yisrael, due to orla, shmitta, teruma and maaser.
• Scotch whiskey--even where it might have been aged in sherry casks.
  Reason Any sherry would be nullified as less than 1/6th. 
  Note Other types of whiskey may not be kosher because:

  • Glycerine may have been added;
  • The whiskey may have been owned by a Jew during Passover in a previous year; or
  • Milk, or alcohol derived from milk, might have been added.
• Sugar (confectioner's) needs kosher supervision only for Passover. Regular sugar never needs kosher supervision (currently).
• Unprocessed foods such as
  • Raw fruits and vegetables (but might need to be checked for insects), and
  • Water, but some unfiltered tap water might have tiny creatures in it which make the water non-kosher.