The "pre-ban" barrels will normally have a bayonet-lug and a threaded muzzle. A "post-ban" will not have the bayonet-lug and will have a crowned muzzle, no threads.
I think for the most part it would be the threads for a flash hider, if the front sight is on it, might also have the bayonet lug also. Now that the ban is over I dont know if it makes much difference now, but it used to be you couldnt put pre-ban parts on a post-ban rifle. If I am wrong I am SURE I will be corrected!
Some states, like New York, continued some form of the AWB after the Federal version sun-set. If you don't live in an effected state, then the term "pre-ban" and "post-ban", related to barrels, is purely a description of the barrel's configuration.