Hi, Jaegermeister,
On second thought, I agree with you. I would probably just give it the "eyeball" test and fire it, but then my eyeballs are pretty experienced, and I am pretty familiar with the various wildcats in that caliber. So a look in the chamber would be enough to rule out anything exotic with a big belted case, and any 6mm based on the 6mm Remington case would be no problem.
Still, it is better to be safe than sorry, and any gunsmith can do a chamber cast.
Jim