To be bang to rights means to have irrefutable proof of wrongdoing, which in this sentence is the act of appearing to own a stolen horse.
Context: "But on the subject of appearing to own a stolen horse, he was caught red-handed and had no way to defend himself. " Claim: "But on the subject of appearing to own a stolen horse, he had rincewind bang to rights and, also, a foot on his neck."
Considering the context, is the claim true, false, or unknown?