Getting a Headshot in CS is pretty easy, especially at close range with medium recoil weapons. Some people Headshot more often than not because of their skill (is there an Autocentre in CS like UT?). However sometimes you see people that ALWAYS headshot even when you walk up behind them and shoot first or the second you go around a corner. Some of these people are probably extrememly skilled, others may well be cheating. Cheating is rife in CS. Some cheats are funny, like the chiken skin cheat, which is a cheat nonetheless.The worst are the ones you can only guess at like Autoshoot, autoaim etc.
Basically by definition: If you play CS on open servers you will encounter lots of cheats. Don't let it get to you or you will start to suspect cheating everytime somebody pulls off a good shot. After you have played a while you will be better able to gauge probable cheats.
The best foil to cheats is to get reallly good. How sad would you feel if you were loaded up on cheat tools etc and some non-cheat was kicking your ass (or would you just assume they were a better cheat?)
Cheats suck, but they will proliferate wherever they can, Sadly Valve have performed badly on this front