The barrel should be marked with the caliber. It's very hard to find any lowers with a special caliber mark on them. Most everyone interchanges their upper assemblies in various calibers with whatever lower is handy for their own personal use.
Now if you were going to get in the manufacture and sale of the complete rifle, you would want to find out what keeps the BATF happy. On second thought, I've never met a BATF agent who I could describe as happy, but they have been polite if not quite to pleasant. Maybe when they get their new boss he can revive their spirits. Now, get rid of the unconstitutional '86 ban on automatics, and I'll smile enough for all of them. Actually, I'll probably need to get my own zip code to handle all the form 1's.