Is My Dog a Purebred Pit?

My 5 month old red nose puppy makes me wonder if she's a purebred or a mutt? Her head doesn't grow at all, it's almost the same as when I first got her at the age of 3 months. She has a slim long body and her muscles are well build. She's like more a game dog instead a bully type, with a big head and bones. I uploaded a photo to tell me what you think since there are some people on there who truly know about Pit Bulls. Don't let the photos fool you. In the photos the head might show bigger than it really is. It's true size is like a tennis ball!