
There is no way of knowing what the pup is, my concern with any sort of rescue bull breed x pup is that you don't know what it will grow up to look like, and it could end up contravening the dangerous dogs act - i.e. look like a pitbull type and require to be registered, neutered, insured (which for a pit would be extortionate) and muzzled.
I guess an older pup of 6 months + would give you a better idea of what it would look like, what height and weight etc, but you still won't ever know what breeds went into it's makeup.
There is a DNA test for dogs that is supposed to tell you what breeds they are, but it's not very accurate and has thrown up some bizarre results when owners actually know what their dog is :-)