How can you tell your lab is pure bred?

We got her yesterday and I noticed these little moles on the back of her mouth with whiskers coming out of it and I knw that pit bulls have those.... Her mom either got pregnant by a pit or a black lab and the owner of the mother swears it was the lab cuz she wouldn't let the pit come near her but always let the black lab so I was just wanting to make sure how do you knw for sure??? All the puppies were born solid black and couple have little white spots on their chest thanks for your help and time!