Don't answer amen if you finish a prayer or blessing within 2.5 seconds after the prayer leader (cantor) finishes the same prayer or blessing as you are saying. However, in these three cases, you may say amen, even if you finish exactly together with the leader:

• End of yishtabach;
• End of Hallel;
• Ma'ariv's shomer amo yisrael la'ad.