how did mendel make sure that the pea plants were true breeding

Mendel performed normal breeding of pea plants and observed only single trait at one time. From the progeny of these plants, he selected only the plants that had the trait that he was looking for. He bred them with each other (F1 x F1) and continued their self pollination for several generations until no other trait showed up. This ensured him that the pea plants were true breeding.