The best way I've found to detect aimbotting, is to have an admin specing+demoing on them, and have zombies approach from all directions. I saw one person who was sneaky about it (would have it on and get like 10 headshots in a row, then turn it off and miss like crazy for a bit, then more headshots) but at one point 3 zombies attacked from 3 different sides (on hideout) and with magnum turned and headshotted all 3 in less than 4 seconds. Impossible without an aimhack =P