>>1
That depends on what the hacking is. Stuff like making the player run faster than physically possible is detectable, but stuff like seeing through walls, or parts of the map that are not yet exposed is implemented entirely on the client side. In FPS games, you might be able to detect movement patterns for auto aim, but an aim bot could always emulate a good human player more realistically. For the hacks that give the player extra knowledge, you'd have to detect the playing continuously acting on this knowledge, over and over again. But how do you differentiate this from luck? Or maybe they just know the game really well, and they can make a good guess for the missing information? Maybe the player isn't hacking at all, but is just really really good at the game? In either case, it will be frustrating for the other players that are constantly defeated by ey, and ey will likely be accused of hacking due to ey's performance.
I heard that in some tournaments, participants are required to run software that monitors all activity running on their computer while they play, so that tools for hacking can be detected. I see this as a violation of privacy though.