While you could have thought that there's nothing to lose by answering amen if you didn't hear the bracha, the gemara says completely the opposite. Shockingly, the Gemara Brachot 47a states that one ensure not to answer Amen without having heard the bracha, termed by Chazal as an Amen Yetoma. Moreover, the gemara says not only is it forbidden, but if you do it, there is a curse that such a person should pass away, leaving his children as orphans! What could possibly have prompted Chazal to consider an Amen Yetoma as such a grievous crime?
In order to address our question, perhaps we can gain some insight from seeing how the Rishonim defined the parameters of Amen Yetoma. Rashi and Tosfot ask that the Gemara Sukkah (51b) seems to be explicitly against the Gemara Brachot. The Gemara Sukkah tells how there was such a great multitude of people in the shul of Alexandria that many people couldn't even hear the Shaliach Tzibbur. To facilitate people answering Amen, the gemara says, the Shaliach Tzibbur would wave a flag as he finished the bracha so everyone could see that they should answer Amen. Seemingly, this gemara takes it for granted that it is permitted to answer Amen even if one didn't hear the bracha. Rashi and Tosfot both answer that answering Amen is only an issue if you don't know which bracha was made or if you don't know if someone made a bracha at all. If you know that someone made a certain bracha, however, even if you didn't hear it, you can say Amen. That's why the people of the shul in Alexandria were able to answer Amen even though they didn't hear the bracha.
Based on the understanding of Rashi, one could suggest that the reason saying Amen without knowing which bracha was made is so severe is because Amen is meant to be a statement of affirmation in the truth the bracha spoke about Hashem. If you don't know which bracha was made and you still say that you affirm its validity, your words become meaningless. Moreover, your intended praise of Hashem turns out to be hollow and without understanding. That's why, says the Maharal, unlike a bracha which is valid if said without understanding as it is intrinsically meaningful, however, an Amen is a statement of Emunah which is useless without understanding.
- Chazal viewed the recitation of Amen very highly and compared its recitation to a signature that attests to the validity of a document. In fact, Chazal tell us that responding Amen is of greater significance than reciting the Beracha. The failure to recite Amen is considered a gross transgression, while responding Amen with great concentration opens the gates of Gan Eden. 
- The letters of Amen are the root letters of the word Emunah, belief or trust. By responding Amen one declares: "I believe in the blessing that I have just heard and I affirm its truth." Additionally, when responding Amen one should have in mind the beginning of the Bracha, "Baruch Atta Hashem", that Hahshem's name is Blessed. Thus, for example, when responding Amen after "Magen Avraham", one should have in mind, "The Name of Hashem should be blessed, and it is true that He shielded our forefather Avraham, and I believe it". 
- One should answer Amen to any blessing one hears whether he wishes to fulfill an obligation, or even if one overhears a Beracha. One should respond Amen after each line in Bircas Hamazon that begins Harachaman. Moreover, the obligation to respond Amen even applies to a Bracha that does not contain Hashem's Name, such as answering to a Mi Sheberach. 
- The proper intention of the word Amen changes with the Beracha. When answering Amen to Birchot HaMitzvah or Birchot HaNehenin (Berachos on enjoying things), one's intention should be to affirm the truth of the Beracha and his belief in it. When answering to Birchot HaShevach, one should have in mind that he is affirming the truth of that praise. When responding Amen to Tefillot one's Amen should be an entreaty to Hashem to fulfill that prayer. At times, Brachot can have multiple purposes and as such one should have multiple Kavanot. 
- When reciting Kiddush on Friday night, we say the words "ויהי ערב ויהי בקר" quietly before saying "יום הששי". In truth, ויהי ערב is actually the second part of the Passuk which precedes יום הששי. We don’t say those words out loud because the first letters of יום הששי ויכלו השמים form the name of Hashem. While we generally avoid reciting Pesukim in ways which differ from their presentation in the Torah, nonetheless we only say the second part, and don't say the first part of the Passuk quietly because Chazal understand the words "טוב מאד" as a reference to death. 
- Rashi (Brachot 47a s.v. Yetoma) and Tosfot (Brachot 47a s.v. Amen)
- Rav Soloveitchik in Reshimot Shiurim (Brachot 47a, p. 501 s.v. VeNirah) explains that Rashi and Tosfot hold that Amen is a function of expressing Emunah in which case only knowledge of the bracha is necessary. The Rabbenu Yonah, however, understood that Amen is a way of accepting the bracha upon oneself, in which case, having knowledge of the bracha without hearing its words isn't sufficient.
- Netivot Olam (Netiv HaAvoda ch. 11; Sifrei Maharal Edition v. 1, p. 112)
- Gemara Brachot 53b, Chaye Adam (Klal 6:1), Gemara Shabbat 119b, Rashi Shabbat 119b s.v. BeChol
- Shulchan Aruch 124:6, Mishna Brurah 124:24
- Shulchan Aruch 124:6, 189:5, 215:9
- Shulchan Aruch 124:6, Mishna Brurah 124:25
- Tanit 27b, Megillah 22a. "כל פסוקא דלא פסקיה משה אנן לא פסקינן"
- Rama 271:10, Levush 271:10, Aruch HaShulchan 271:25, Chatom Sofer OC 10