How to detect an humanized bot and a wallhack.

[Sky]

They keep shooting at a target trough gas (Don't forget some people use cg_wolfparticles "0" to see trough smoke, but it doesn't work for the gas/smoke grenades) You can see the gun shot of your enemy and then aim for the head also from large distance real easy.
They headshot in impossible ways (like from a very high distance, where you can't see the enemy good)
They often switch from a player to another (mostly the nearest player) without reason (sometimes do this but what you exactly mean with no reason.

But it's a good tutorial for some admins(players)

Well I putted it in bold because there's no 'real' reason if it would be a real players aim. BUT like Destroy said, the reason is because they often aim at the nearest player. So when a player comes closer it switches.