What Breed Is My Dog?

I recently got Molly from a rescue centre. At the time I was told she was a Boxer/American Bulldog (Bulloxer), but I have a sneaky suspicion she's a Pit Bull mix. I'm hoping someone with more experience with these breeds can enlighten me so I can give her the best care and address a couple of behaviour issues.

She is approximately 6 months, stands 21 inches at the shoulder, and weighs exactly 50 pounds (23kg).
Thanks in advance for any help.

By stefworld