Even if one did not hear a Beracha recited in its entirety, and heard only the end of the Beracha, he nevertheless recites "Amen." Thus, for example, if somebody is reciting "Asher Yasar," and another person comes along and hears him conclude, "U’mafli La’asot," he answers "Amen."
Hacham Ovadia Yosef adds that even if a person did not hear the Beracha at all, but he saw a person reciting a Beracha and he knows with certainty which Beracha is recited, then he answers "Amen." For example, if a person saw somebody leave the restroom and mutter to himself, it is clear that this individual recites "Asher Yasar," and so once he finishes, the other person answers "Amen," even though he did not hear the Beracha at all. Similarly, if a person sees somebody holding a fruit, mutter some words, and then bring the fruit to his mouth, he knows for certain that this individual just recited "Boreh Peri Ha’etz," and so he recites "Amen."
Hacham Ovadia ruled that one answers "Amen" to a Beracha which he heard over a live broadcast. Although one does not answer "Amen" after hearing the recording of a Beracha, he does answer "Amen" if he hears the Beracha live, either on the radio, by phone, or through other technological devices.
Summary: One answers "Amen" even if he heard only the end of a Beracha, and even if he did not hear the Beracha at all, but he saw somebody recite a Beracha and he knows which Beracha was recited (such as if he saw the person reciting a Beracha over an apple, or after leaving the restroom). One does not answer "Amen" after hearing the recording of a Beracha, but he does answer "Amen" to a Beracha he hears over a live broadcast.
