The Shulhan Aruch (209:3) establishes that whenever one is required to recite a Beracha, and he cannot remember whether or not he recited it, he does not then recite the Beracha. Since all Berachot are required Mi’de’rabbanan – by force of Rabbinic enactment, and not on the level of Torah – we follow the famous rule of "Safek De’rabbanan Le’kula," which allows assuming the more lenient possibility when faced with a doubt concerning a law enacted by the Rabbis. Since Berachot are required only Mi’de’rabbanan, in cases of uncertainty we are lenient, and do not recite the Beracha in question.
However, the Shulhan Aruch makes one very important exception to this rule – namely, Birkat Ha’mazon. If a person ate bread to the point of satiation, then he is required to recite Birkat Ha’mazon on the level of Torah obligation, as the Torah explicitly commands, "Ve’achalta Ve’sabata U’berachta" – "You shall eat, be satiated, and bless" (Debarim 8:10). Therefore, if a person ate to the point of satiation, and is then uncertain whether or not he recited Birkat Ha’mazon, he must now recite it, because a Torah obligation is at stake.
This Halacha applies only to men. Women are obligated to recite Birkat Ha’mazon only "Mi’de’rabbanan," and so if a woman was required to recite Birkat Ha’mazon and then cannot remember whether she did so, she does not recite it.
The Halachic authorities debate the question as to whether a person in such a case recites all four Berachot of Birkat Ha’mazon, or only the first three. The fourth Beracha of Birkat Ha’mazon ("Ha’tob Ve’ha’metib") is not included in the Torah obligation, and was added by the Sages. Seemingly, then, in a situation of doubt, only the first three Berachot should be recited, and not the fourth, since only the first three are required on the level of Torah obligation. Nevertheless, the Peri Hadash (Rav Hizkiya Da Silva, Italy-Jerusalem, 1656-1695), cited by the Hid"a (Rav Haim Yosef David Azulai, 1724-1806) and the Mishna Berura (Rav Yisrael Meir Kagan, 1839-1933), writes that if a person is uncertain whether he recited Birkat Ha’mazon, he recites even the fourth Beracha, since it has been established as part of Birkat Ha’mazon. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), however, writes that we should follow the standard rule of "Safek Berachot Le’hakel" and not recite this Beracha, which was instituted by the Rabbis. This is the view accepted by Hacham Ovadia Yosef, in both Hazon Ovadia and Halichot Olam. Hacham Bension Abba Shaul (Israel, 1924-1998) writes in Or Le’sion (13:4) that a person in this case should preferably think the text of the fourth Beracha in his mind. The Rambam (Rav Moshe Maimonides, Spain-Egypt, 1135-1204) ruled that thinking a text is akin to reciting it, and so thinking the fourth Beracha can be a solution for somebody facing a situation of doubt.
Another question arises in the situation of one who ate to the point of satiation but did not drink at all during his meal. Some Rishonim maintained that even if one eats to the point of satiation, he is not obligated to recite Birkat Ha’mazon on the level of Torah obligation if he did not drink anything. Accordingly, the Ben Ish Hai (in Parashat Hukat) and Hacham Bension Abba Shaul (13:6) maintain that if a person ate a meal but did not drink, and he then cannot remember whether he recited Birkat Ha’mazon, he does not recite Birkat Ha’mazon, since his obligation applies only Mi’de’rabbanan according to some opinions. However, Hacham Ovadia Yosef strongly disputes this position, noting that only a very small minority of Rishonim maintain that the Torah obligation does not apply if one did not drink, and this view was rejected by Maran (author of the Shulhan Aruch) in Bet Yosef. Therefore, even if one did not drink at all during his meal, he must recite Birkat Ha’mazon if he had eaten to the point of satiation and now cannot remember whether or not he had recited Birkat Ha’mazon.
This Halacha applies only in regard to Birkat Ha’mazon, and not to Beracha Aharona. The Shulhan Aruch writes very clearly that Birkat Ha’mazon is the only blessing required on the level of Torah obligation, and is thus the only blessing recited in situations of doubt. Therefore, if one is required to recite "Me’en Shalosh" or "Boreh Nefashot," and he cannot remember whether or not he recited the Beracha, he does not recite it.
There is a dispute among the Halachic authorities regarding the case of one who cannot remember if he recited Birkat Ha’mazon after eating a quantity of "Mezonot" food that requires Birkat Ha’mazon. Generally, of course, one who ate "Mezonot" food must recite "Al Ha’mihya." However, one who recited a large quantity – 8 oz. – of "Mezonot" food is required to recite Birkat Ha’mazon, and one who knows he will eat this quantity must wash Netilat Yadayim and recite "Ha’mosi" over the food. Some Poskim were of the opinion that Birkat Ha’mazon in such a case is required only "Mi’de’rabbanan," as the Torah obligation applies only after one eats bread. Others disagree, and maintain that Birkat Ha’mazon after eating 8 oz. of "Mezonot" food is required on the level of Torah obligation, and this is the view accepted by Hacham Ovadia Yosef (Hazon Ovadia, p. 234). Therefore, if a person ate this quantity of "Mezonot" food and then cannot remember whether he recited Birkat Ha’mazon, he must recite it.
Summary: If one was required to recite Birkat Ha’mazon but cannot remember whether or not he fulfilled his obligation, then if he had eaten to the point of satiation, such that the Birkat Ha’mazon obligation applies on the level of Torah law, he must recite the first three Berachot of Birkat Ha’mazon. Preferably, he should then think the words of the fourth Beracha in his mind. This applies only to men; a woman who is uncertain whether she recited Birkat Ha’mazon does not recite it. If one was required to recite a Beracha Aharona and cannot remember whether or not he fulfilled his obligation, he does not recite the Beracha.
