When someone hears another Jew recite any blessing (Berakha), even if he is not obligated in that specific blessing, he must answer Amen. Not only that, but even if he didn't hear the entire blessing, from beginning to end, he must answer Amen. In Midrash Ruth it says very harsh things about one who does not answer Amen after a blessing, and one must be very careful about it.
Having said that, there are times that one should not answer Amen to another person's blessing. If the person reciting the blessing is an Epiqoros (lit. heretic), even if he is Jewish, or a Kuthean, or an 'Akum (idol worshipper), then one does not answer Amen.
Similarly if a small child is learning to recite a blessing, we do not answer Amen. Also, if someone changes the formula of the blessing, we may not answer Amen to it.
(See Shulhan 'Arukh, Orah Hayyim, 215:2. Kaf Hahayyim, ibid., Oth 9)
when not to answer amen, brachot, brochos, berakhoth