Answer:
You can tell if your American Pit Bull Terrier is a purebred if it has papers from a reputable kennel club or pedigree service. Otherwise, you'll have to look up the standard from www.ukc.org (as the American Pit Bull Terrier is not an AKC recognized breed) and compare and contrast your dog to it. You can also buy a DNA kit from wisdompanel.com, although it could be costly. You could also have a reputable breeder or your vet look your dog over. It doesn't really matter if your dog is a purebreed though, as long as it gives you love and makes you happy, that's all you need. :)