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 (public prayer), or to all Berachoth?
If you notice someone mumbling a Berachah on an apple or a cookie, and you either hear part of the Berachah or you can just tell which Berachah is being said, or you notice someone intently saying a long Berachah after leaving the restroom, does the above rule from the RaM"A mean that you can answer Amen in these situations, even if you didn't actually hear the Berachah being said? Can a deaf person then also answer Amen to Berachoth and Tefillah BeTzibur?
If so, would it then follow that if you are in Shul and you hear someone conclude their Berachah (eg., "Yotzer Or"), that you have to respond Amen (assuming you are at a point in your own Davening in which you are allowed to respond generally to Berachoth)?