Well, a certain deal of truthfulness needs to be considered if the Predator comes from a society that highly praises prize kills. The Predator in question could kill any random human, remove their skull, and play it off as a worthy kill in their collection without another Predator present. It's the same deal with the etchings. Considering that the Alien blood would still have DNA and whatnot, it would probably just take a quick test on their part to determine whether it's really Alien blood.