If you forgot to say “Mashiv HaRuach U'Morid HaGeshem” in winter and realize before finishing the blessing “Mechayeh Hameisim,” say it right where you remember. If you don't realize until after finishing that blessing, you must return to the beginning of the Amidah. If you're unsure whether you said it: after thirty days [from the switch], assume you davened correctly as usual; within the first thirty days, you must go back and repeat. (Kitzur Shulchan Aruch 19)