If a person doesn’t hear the end of a beracha his friend (or the shatz) is making, he may still answer amein, provided that he knows which beracha is being concluded. It is forbidden to answer amein without knowing to which beracha it applies.
Shabbos Bulletin Parshas Behar 5778
About the Author: Lev Seltzer
Lev Seltzer has been a member of Beis Tefillah from just a short time after its founding. He currently manages the BTYA.org website, and can be found every morning in the BTYA "Vasikin Room" where he is the gabbai of the minyon.