You'll know you've been banned because on the private lobby screen you'll see "*FairFight Cheat detected." When you go to join a game, only those that have also been banned will be able to find you, so hopefully your aimbot is better than theirs.
However, what happens if you try and join a game with your pals? You can play with them, but it'll treat them all as banned too for that session.
Unfortunately though, if you've been banned for a real reason, chances are you aren't going to be able to be unbanned and will need to buy another copy of the game if you want to get back in the action against non-cheaters.
Still, if you feel you've been banned unjustly, you can email email@example.com to request a reinstatement.