If one forgets Ya’aleh Veyavo in davening, but remembers before saying Hashem’s name at the end of Hamachazir Shechinato L’Tziyon, he goes back. If he remembered between the conclusion of that Brachah and Modim, he recites it at that point, without going back. If he remembers any time after that point, but before taking three steps back, he returns to the beginning of Retzei. If he remembers after taking three steps back, the Amidah is repeated.
If one is unsure whether he recited Ya’aleh Veyavo, the Amidah is repeated.