Peeta found out that Katniss was pretending to love him for the parachute pressies this event doesnt actually happen in the film ! :) xx
In The Book, The Hunger Games, Outside The Train On The Way Back To District 12, Peeta Found Out That What?
Peeta found out that Katniss wasn't really in love with him, he found out it was all a show.
Katniss had been (partially) faking her love for him "for the cameras" so they could get food and medicine from the sponsors. She was just trying to survive.
He learned that the entire time in the arena and in front of the cameras, when she was being romantic, it was all just a show. She really wasn't in love with him.