More anti-cheat developers are adding various types of detection for aimbotting. Fairfight uses stats-based tracking to detect aimbots, and will catch you eventually if you aimbot for long enough. 
CS:GO uses an overwatch system for ranked play where other players can review your gameplay. 

If you want to aimbot, pay attention to the following things:

For our own cheats we've developed safer default aimbot settings which already take these settings into account, to help protect our members.

With more aggressive anti-cheat around now, it's very important you follow the instructions for your cheats. Opening a cheat program while a game is running can result in an instant detection, which is why for games with anti-cheat like Easy Anti-Cheat or Battleye you'll almost always have to load your cheat from the cheat client before starting the game.

If you never die, and always make a bee-line to the next opponent following them where-ever they go, you'll inevitably rank up player reports as your opponents will assume you're a cheater just due to your performance. 

In some games this doesn't matter, but in others it can result in a game admin reviewing your account which can result in a ban. It's worth taking a loss once in a while and playing as though you don't know exactly where an opponent is.