Eh, the only way you can prove they are not mutts is with export papers of the boa itself, or from all of their ancestors which a lot of breeders don't even bother keeping. In the hobby, it's "if it look like something, then it is that." There are a few people that do care about it like Vin Russo and Hermann Stöckl who does keep track of stuff like that.
Yeah... tell that to all the Hogg Island mutts that exceed 6'.... they look like pure Hogg, but they are not. For instance, if I ever breed my Nicaraguans, I will sell them as Central American because I don't have the documentations proving their origin, and is unsure of their genetic background. They look Nicaraguan, but are they Nicaraguan? I have my doubts knowing the ethnic morals of most people.
Edit: Anyway, boas come in all different colour now. Not only with mixed locality, but also with morphs as well.