quas is close, but kinda wrong.
As long as you have NO reason to suspect him of not being allowed to own. You know he is old enough, you have not heard of him being a felon, was not told he was a felon or otherwise incapable of owning, you dont think he is crazy, etc etc.
The only way to KNOW, would be to do a Background check....which would probably involve an FFL....
So, you don't need to know, you just can't have any reason to think otherwise.