I am fairly new to AR's. How do I identify a pre-ban reciever? For instance, if I am at a gun show and someone is trying to sell me a a rifle assembled with a bushmaster reciever as a pre-ban rifle how do I identify that is in fact a preban rifle and not actually a post ban reciever with pre-ban upper...

I guess what I am getting at... could someone basically take a post ban reciever and build it as a pre-ban rifle and then pass it off on someone like me who probally wouldn't know the difference? How do make sure this does not happen.

Thanks Robert

Sadly, this probably happens every day.  The starting point is knowing the serial number ranges.  If the receiver wasn't made before the ban, it cannot be a grandfathered pre-ban no matter what the seller tells you.

If the receiver was made before the ban, you may want some sort of proof that it's grandfathered.  

ATF suggests that dealers get a signed statement from the owner of the rifle on or before 9/13/94 that the weapon was assembled or set aside with all parts necessary to make it a complete rifle.  

No such guideline is given for private buyers, and strictly speaking you don't HAVE to have any proof.  I think a letter like the one recommended for an FFL is a good idea, though, in case you ever wind up in court having to prove the defense that your other-wise illegal rifle is a legal grandfathered pre-ban.

In their "FAQ"....