There is dna testing that can be done to determine that. Talk to your veterinarian to see if they are affiliated with a laboratory that can perform this dna testing for you.
I have a red nose pitbull,he has a black muzzle and a little white on it.how do i tell if he's a full blooded pit?
You can check tha snout pit bulls have shorter snouts tha ears should fold over boxed head
My best guess would be to contact the person you got your dog from for this question