Crp, the serial number is the best way.....but not always the right way. Some manufactures sold lowers (incomplete rifles) and dealers sold them retail. If a lower was logged into a dealers A&D book and was there after the ban---then sold, it cannot be considered a preban-ever! If it was sold prior to the ban, then who can prove it was never built into a complete rifle prior to the ban.......my guess is no one. Colts and Bushmasters are the easiest to trace back, followed by Armalite and several others. Some can be recognized by color, as some early companies had gray receivers. Just takes a little time to track them down, but not hard to do. Also remember.....there were two (2) bans, first in 1989 for imports.....then the second for domestic`s. I would probably take a hard look at the one you are looking at, and check the serial number via the internet......could be a great buy if correct and preban.