If one is unsure whether he recited Asher Yatzar, what should he do?

Hello, 

One should not recite it, because in a case of doubt regarding blessings, one rules leniently. However, you may recite it without mentioning the name of Hashem, and you should think the name of Hashem in your heart. 

Sources: Based on what the Shulchan Aruch, Orach Chaim (siman 209, section 3) writes: if one is uncertain whether he recited a blessing, he does not recite it, neither at the beginning nor at the end, except for Birkat Hamazon, because it is from the Torah. This is likewise explained in the Birkhei Yosef, Orach Chaim (siman 7, ot 3), in the Machazik Berachah (there, ot 1), and in the Aruch HaShulchan (there, section 5) regarding a doubt in the blessing of Asher Yatzar. So too writes the Ben Ish Chai (year 1, Parashat Vayetzei, ot 12) that if one is doubtful whether he recited Asher Yatzar, he should not recite it with Hashem's name and kingship, but should think Hashem's name and kingship in his heart. See also what is brought on this in the book Shegagot Mi Yavin, part 1 (siman 2, section 6 and the note there). 

With blessings, Hillel Meirs