This topic comes up every couple months, always causes lots of knashing of teeth, but it's fun to watch...kinda like a freight train wreck.
The only folks that can tell you how your frame was manufactured is the T/C Factory. Their list is NOT provided as a matter of course to any federal or state agency. If the ATF calls them, they'll tell them how a certain serial number was manufactured and registered when built. Key here is T/C pays different taxes to the feds based on what they manufacture so they keep good records vice just calling everything a handgun frame, which would be best for us because the NFA doesn't put an upper limit the length of barrel on a pistol.
So if you're buying a new complete gun and it's coming from the factory (not changed by the local dealer) it will be assembled how it was manufactured and in the appropriate box so labeled. If you're buying a used frame, you can call T/C and they'll tell you how it was manufactured. By the letter of the law, what Lone Hunter said is absolutely correct. If it was manufactured as a rifle frame, if you put a barrel less than 16" on it, you have "made" a short-barreled rifle and it is subject to the National Firearms Act (doesn't matter what stock is on it). To be legal you must advise ATF of your intentions to "make" a NFA regulated firearm, pay a $200 fee and wait for their permission before you 'make' it. (all this is on their web site, not TC specific because they were thinking rifles with cut off barrels and stocks when they wrote the law. You know the kind the gangsters use, and they weren't going to follow the law anyway).
Now like 444encore said, why are they going to ask? An agent looking at a T/C assembled properly as a pistol or a rifle has no reason to suspect the frame on that pistol is really a rifle frame...can you say no probable cause? Now if an ATF agent sees you with a carbine stock and a less than 16" barrel he can and WILL stop you and you WILL be sorry because the NFA regulated firearm is in plain sight (so unless you registered your short barreled rifle he's gonna put the hurt on ya!). But barring that, you'd really have to make the ATF mad to have them take your serial number, get permission and then call T/C to find out how your frame was manufactured. Sounds like a frustrated investigation that they couldn't get you on what they wanted to and started looking for any way to get you. I'd read the ATF website if you really want the straight skinny. It's boring, but it's informative...kinda like directions that come with electronic gizmos, I don't read them until I can't figure out how to make this @#$% thing work. Guess that's why the clock still flashes on my VCR and I can't make the blasted Emoticons work today!
Bill