According to this answer citing RaM"A (124:11), you can answer Amen to any Berachah even if you don't hear it, so long as you know which one is being said. Does this apply only to Tefillah BeTzibur ...
I find that most people have tendency to say most blessings (on food, going to the bathroom etc.) quietly to themselves. This may preclude others from having a chance to answer amen afterwards. Is the ...