I think the best thing you can do is to require GLS login. So many more hackers are tracked down and banned because of it. (Thanks PicaSSo)
What to look for:
+Players shooting in places where enemies are that don't seem logical (wallhackers)
+Players not jumping or strafing and still going faster than all the others (speedhackers)
+Banned users evading bans (check ban list often)
If you see any of this, get their name, IP, and the time (and other names used if ya have any) and post them here along with a firstperson demo of the offense
Cheers,
y00tz
p.s.
Thanks for your help