Well, if the person you got the dog from said the dog was a mix, then that person is not actually a breeder. Responsible breeders do not breed mixes on purpose because there are already millions of mixed breed dogs who do not have homes and it is not right to make more of them.
What this means, essentially, is that you cannot trust what that person said the dog is, unless that person owns both parents, and of course if those dogs are mixed breed then they still cannot tell you accurately.
You can do a DNA test, it might tell you, but I have heard those are not always accurate. I know human DNA tests are not always accurate either, because I did one and it was pretty far off from what I absolutely know to be true.
The only thing that really matters is if you love your dog. If you do, great. But if the dog ends up looking like a pit bull, it would be very much in your best interests to make sure the dog is meticulously trained and never gives anyone a reason to fault your dog for being a bully breed looking dog. There are a lot of prejudices against pit bulls.