Anyone who hears a blessing from beginning to end, thereby intending to fulfill his duty, has fulfilled his duty even if he has failed to answer Amen.
Whoever answers Amen after a blessing recited by someone else is regarded as if he has recited it himself, provided that the person who recited the blessing is obliged to do so.
Rambam/Maimonides Mishneh Torah Yad Hazakah Rabbi Moses Maimonides (1135-1204) Chapter 1:11 page 61