If you have finished the amida's yihiyu l'ratzon(but have not yet finished the segment from Elohai netzur through u'chshanim kadmoniyot), you may reply to:

• Bar'chu,
• Kedusha,
• Amen to ha'el ha'kadosh,
• Amen to shomei'a tefila,
• Modim anachnu lach (just those 3 words),
• Birkat cohanim, and
• Kaddish.

However, you may not say anything else (such as other amens) until you have completed the entire amida (including the last word, kadmoniot).