The halacha is that a person who has a safek if he said retzei on shabbos needs to repeat benching because we assume that he did not say it.
Would the halacha be different for a person who eats bread only on Shabbos, and never during the weekdays, in which case perhaps we should assume that he did say it since he never says benching without retzei?
Good question. Logic dictates that if he only says birkas hamazon on Shabbos with retzei, therefore not considered a sofek and does not have to repeat. Similarly if a child tells him that he said retzei since we have a גילוי מילתא that he recited retzei, it is sufficient.
O:CH 114:Mishnah Berurah 38, 188 Mishnah Berurah16, 422 Mishnah Berurah 10, Shevet Halevi 4:18-2