The only two cheats I feel I have seen so far on COD are wallhacks and fullbright hacks. Both of which can be done at graphics driver level so are hard for COD to spot.
Some people moan about the KillCam, but its vital in keping this sort of thing under control. Most cheaters are just too stupid to mask their actions. You can see wallhack users tracking people behind walls etc.
I also have a demo I took of a COD player I was convinced was using a fullbright hack. This is harder to prove as he/she is either blessed with eyes that can spot a moving pixel and evaluate that its an enemy in 10ms. Or (as I think is likely) those 4 pixels were glowing bright green against the muted tones of the map. That game also showed the other tell-tale cheat effect; the player scored over twice what anyone else on the server did (and the server was not just full of old gits like me
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