How Can I Tell If These Dogs Are Purebred Pit Bulls?

I'm looking for a Pit Bull. I'm hoping these two parents of such puppies are indeed Pit Bulls as the breeder is telling me.

The breeder says the mother (blue) is a blue nosed Pit Bull and the father (brindle) is also a Pit Bull. The puppies are also shown. The black one (female) is the one I'm getting.