When one hears a brachah recited by someone else, they should answer “amen”, even if they did not hear the whole brachah from beginning to end. When answering amen one should have in mind that it is true (this is the literal definition), and that they believe in what was just said. During chazaras hashatz by the middle brachos of shmoneh esrei one should also think that what was just said by the chazzan should be fulfilled.