How can I tell if my English Mastiff is a purebred?

A lady that my husband works with gave us a dog she claims is a purebred mastiff. She left her abusive husband and moved into an apartment. He called her to tell her about three weeks later that he was moving out and he left the dogs at the house they had together and she needed to find a home for them. She talked her landlord into allowing her to keep the Chihuahua but he said no on the English Mastiff. She said her husband took all of his belongings and the papers must have been in there with the stuff. We don't know her very well, so she could be telling us a lie. She claims her husband paid $650 for her. The dog does look like a purebred mastiff (very big, tall with huge feet, fawn color with a dark face and ears) but is there a way to tell without an expensive test? Like markings or anything specific to mastiffs? She is 8 months old and she is HUGE!! Weighing in at 80 lbs.