I haven't seen it myself but apparently the most commonly used hack is a speed hack. I've seen in other games and servers I've admin'ed that they have a speed hack detector, which normally they just set it to display a message:
e.g. "Player urraka is moving at _______"
Here's the link and info for the other source games if you're curious
https://forums.alliedmods.net/showthread.php?t=131219 , I saw it most commonly on tf2.
Essentially it detects how fast they are moving and if they're moving over the normal speed for the game. Curious to see if someone could make a mod that server owners could use or even the devs could implement it into the game themselves? Maybe combine this with VAC and you'll be protected