• One should respond Amen whenever he hears another saying a blessing, even if the blessing does not contain G-d's name, such as the "May the Merciful One be blessed..." in the Grace After Meals.
• In order to answer Amen, one may either hear the complete blessing, the ending of the blessing, or at least know to which blessing one is answering Amen to. If he does not hear the blessing at all and does not know to which blessing he will be responding, he does not answer Amen.
• It is appropriate to answer after hearing G-d's name in a blessing, "blessed is He and blessed is His Name" (Boruch hu uvaruch Shemo). However, since this is only a custom and not an institution of the Rabbis, one may not say it during prayers at points where one may not interrupt, such as during the "praises" (Pesukei dezimrah), even though he would be permitted to interrupt for Amen.
• One is permitted to answer Amen to any blessing that he hears through a microphone or live hookup.
• One should answer the Amen immediately after the blessing is completed. One should not answer a "Hurried Amen," that is an Amen which is said before the blessing is even completed. Nor a "orphaned Amen," that is an Amen which was said a while after the blessing was completed. (Alter Rebbe's Shulchan Aruch O.C. 124:1)
• It one completes his own blessing at the same time that another person finishes his blessing, one does not answer Amen on the other's blessing, for it appears as if he is saying Amen to his own blessing.
• One should not answer Amen to an unnecessary blessing. For example, if one washed for a meal and then says a blessing on the vegetable salad (during a meal one does not say a blessing on the various other foods, as it is included in the blessing of bread), that second blessing is considered an unnecessary blessing and no response of Amen is said.
• Amen is said to a blessing that a child says for something which he needs to say a blessing on such as over food before eating. However, if the child is saying a blessing as part of his learning routine, such as when practicing reading, Amen is not to be said.
• If someone says a blessing, but changes the text as instituted by the Rabbis, one does not answer Amen.
• One may answer Amen to a blessing said by a non-Jew, if he hears the complete blessing from beginning to end. If the non-Jew is a believer in idolatry, Amen is not said.
• Whoever forces himself to be a cantor against the will of the congregation, is not entitled to have Amen answered to his blessing (that he says as the cantor).
• One should concentrate well when answering the "Amen Yehei Shmei Rabbah..." in Kaddish, for those who answer "Amen Yehei Shmei..." with the fullest power of concentration will even have their decree of seventy years torn up and given more years of life. Also, this should be said in a loud (not yelling or shouting) voice, for this too achieves the removal of all evil decrees. The text:
Transliteration: Amein. Y'hay sh'may raba m'vorach l'olam ul'ol'may ol'ma-yuh yis-buh-raych.
Translation: May His great name be blessed forever and for all eternity.
• One who ends the last blessing before the morning Amidah prayer, "Go'al Yisroel," together with the quorum, does not answer Amen.
• In the prayers there are certain exceptions to this rule (that one finishes the same blessing together with another, does not respond Amen), such as the blessing of "May Your name be praised..." (Yishtabach). If the person saying Yishtabach ends at the same time as the cantor, he should answer Amen.
• If however, he finishes a blessing at the same time as the cantor, but the blessings are different ones, then he responds Amen to the cantor's blessing.