Determining if a rifle is pre-ban or not is both complicated and simple:
Guns marked "Colt AR15" are exempted *by name* (unless also marked "For LEO/Government Use Only"), so they are automatically pre-ban.
Guns not marked "Colt AR15" had to have been in "Assault Weapon" configuration (as defined by the law), and legally possessed, on Sept 13, 1994 in order to have been "grandfathered" as pre-bans under the law.
Unfortunately, due to the way the law was written, and how the charge would be prosecuted, YOU have to be able to prove the status of your non-named rifle.
So, before you spend "pre-ban" money on a non-named rifle, make sure you have the "proof". This can be a letter from the manufacturer (for some guns) stating its configuration before the ban, or dated receipts, or whatever. But get this BEFORE you spend the money, because if you don't, there's a good chance you won't ever get it, and you'll have paid pre-ban prices for a post-ban rifle.
-Troy