Clearly you also need to do some reading on the relationship between an Amstaff and an APBT.
At the end of the day, if you don't have papers you'll never know whether your dog is purebred or not. You can get some idea by looking at them but it ain't gonna gaurantee anything. I own a SBT without papers, his breeder(BYB) said he was pure but some of his look doesn't quite make the grade ... so like you, i'm stuck in limbo land, not sure. BUT, I don't care. My dog is awesome, i honestly couldn't ask for a better companion.