If there may be any question about the Jewish lineage of a bride-to-be or bridegroom, his or her female antecedents should be confirmed as having been Jewish (born of a Jewess OR halachically converted) back to when a shomeret Shabbat woman has been positively identified (or back as far as possible). As a practical matter, three or four generations may be as far back as most Jews can be traced.
 
Also, make sure that if there was any divorce, that the divorce was kosher, with a get.