Siman 215
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א אֵין עוֹנֶה אָמֵן אַחַר בִּרְכוֹתָיו, אֶלָּא אַחַר שְׁתֵּי בְּרָכוֹת אוֹ יוֹתֵר שֶׁהֵם סוֹף בְּרָכוֹת; וְנָהֲגוּ לַעֲנוֹת אָמֵן אַחַר יְהַלְּלוּךָ וְאַחַר יִשְׁתַּבַּח. הַגָּה: וְיֵשׁ אוֹמְרִים שֶׁאֵין עוֹנִין אָמֵן רַק אַחַר בִּרְכַּת בּוֹנֵה יְרוּשָׁלַיִם בְּבִרְכַּת הַמָּזוֹן, וְכֵן הַמִּנְהָג פָּשׁוּט בִּמְדִינוֹת אֵלּוּ וְאֵין לְשַׁנּוֹת; (תּוֹסָפוֹת וּמָרְדְּכַי ר"פ שְׁלֹשָה שֶׁאָכְלוּ וּמַהֲרִי"ק שֹׁרֶשׁ מ"ב) וּבִמְקוֹמוֹת שֶׁנָּהֲגוּ לַעֲנוֹת אָמֵן אַחַר יְהַלְּלוּךְ וְיִשְׁתַּבַּח, יַעֲנֶה גַּם כֵּן אַחַר בִּרְכַּת שׁוֹמֵר עַמּוֹ יִשְׂרָאֵל לָעַד.
1 Responding "Amen" after blessings. Contains 4 paragraphs: One does not respond "Amen" after his own blessings, except after two or more blessings that conclude a series of blessings. It is customary to respond "Amen" after "Yehallelucha" and after "Yishtabach." Rema: There are those who say that one should respond "Amen" only after the blessing "Boneh Yerushalayim" in Birkat HaMazon, and this is the widespread custom in these countries, and it should not be changed (Tosafot and Mordechai, beginning of the chapter Three Who Ate; Maharik, Root 72). In places where they are accustomed to respond "Amen" after "Yehallelucha" and "Yishtabach", one should also respond after the blessing "Shomer Amo Yisrael La'ad."
ב הַשּׁוֹמֵעַ אֶחָד מִיִּשְׂרָאֵל מְבָרֵךְ אַחַת מִכָּל הַבְּרָכוֹת, אַף עַל פִּי שֶׁלֹּא שָׁמַע כֻּלָּהּ מִתְּחִלָּתָהּ וְעַד סוֹפָהּ, אַף עַל פִּי שֶׁאֵינוֹ חַיָּב בְּאוֹתָהּ בְּרָכָה, חַיָּב לַעֲנוֹת אַחֲרָיו אָמֵן; אֲבָל אִם הָיָה הַמְבָרֵךְ אֶפִּיקוֹרוֹס אוֹ כּוּתִי אוֹ תִּינוֹק, אוֹ הָיָה גָּדוֹל וְשִׁנָּה מִמַּטְבֵּעַ הַבְּרָכוֹת, אֵין עוֹנִין אַחֲרָיו אָמֵן. הַגָּה: וְעוֹנִין אָמֵן אַחַר עַכּוּ"ם, אִם שָׁמַע כָּל הַבְּרָכָה מִפִּיו (הר"י פֶּרֶק אֵלּוּ דְּבָרִים).
2 If one hears a fellow Jew reciting one of the blessings — even if he did not hear the entire blessing from beginning to end, and even if he himself is not obligated in that blessing — he is required to respond "Amen" after it. However, if the one reciting the blessing was a heretic, or a Samaritan, or a young child, or an adult who altered the established wording of the blessings, one does not respond "Amen" after him. Rema: One may respond "Amen" after a non-Jew if he heard the entire blessing from his mouth (Rabbenu Yitzchak, chapter Eilu Devarim).
ג וְהָא דְּאֵין עוֹנִין אָמֵן אַחַר תִּינוֹק, דַּוְקָא בְּשָׁעָה שֶׁלּוֹמֵד הַבְּרָכוֹת לִפְנֵי רַבּוֹ, שֶׁמֻּתָּר לְלַמֵּד לְתִינוֹקוֹת הַבְּרָכוֹת כְּתִקְנָן, וְאַף עַל פִּי שֶׁהֵם מְבָרְכִין לְבַטָּלָה בִּשְׁעַת הַלִּמּוּד; אֲבָל בְּשָׁעָה שֶׁהֵם מְבָרְכִין לִפְטֹר אֶת עַצְמָן, כֵּיוָן דִּבְנֵי חִנּוּךְ הֵם עוֹנִים אַחֲרֵיהֶם אָמֵן; וְכֵן בְּשָׁעָה שֶׁאוֹמְרִים הַהַפְטָרָה בְּבֵית הַכְּנֶסֶת.
3 The rule that one does not respond "Amen" after a child applies specifically when the child is learning the blessings from his teacher, for it is permitted to teach children the blessings in their proper form even though, during the learning, they are reciting the blessings in vain. However, when the children are reciting the blessings to fulfill their own obligation — since they are of an age of education (bnei chinuch) — one does respond "Amen" after them. The same applies when they recite the haftarah in the synagogue.
ד כָּל הַמְבָרֵךְ בְּרָכָה שֶׁאֵינָהּ צְרִיכָה, הֲרֵי זֶה נוֹשֵׂא שֵׁם שָׁמַיִם לַשָּׁוְא, וַהֲרֵי הוּא כְּנִשְׁבָּע לַשָּׁוְא וְאָסוּר לַעֲנוֹת אַחֲרָיו אָמֵן.
4 Anyone who recites a blessing that is unnecessary is considered as one who takes the Name of Heaven in vain, and it is as if he swore a false oath. It is forbidden to respond "Amen" after him.