All you really need is where the character is looking, their location and the terrain map, all of which are things the server has authority over or can check easily.
Distinguishing between a good player and a bot probably won’t be that hard. A simple aimbot would probably fire exactly at a target’s (0, 0) coordinate, while a good player may be a frame or two early or late. Someone with wallhacks will behave differently if they know someone is around a corner. There’s almost certainly going to be small “tricks” like that that an AI can pick up on.