Q. If one did not hear the bracha being said, but he realizes what it was, as when people bow down for moddim or one has an apple in his hand that he is about to eat, can he answer Amen?
A. Answering omein is an affirmation to the brocho recited, therefore if one is unaware of what was said he is actually prohibited to answer omein, as it is considered an omein yesomo or orphaned amen. Remo O.H. 124: 8 -11).
As is mentioned in Talmud (Suka 51b) in regards to the flag signals of the well attended Great Synagogue of Alexandria, we do answer amen even when we did not hear at all the recited brocho, as long as we are aware which brocho is being recited. (See Igrois Moishe O.H. 4: 91: 4, and question 482.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a