It's usually pretty easy for me to tell, being really good doesn't mean you're cheating, unless you're using an aim bot, but the aimbot is not so popular. Its the see-through-walls cheat that people love to abuse, they think its so funny to shoot you through a box, and that is pretty obvious to detect. Even if they're not shooting through a box, you can tell they have extra sensory perceptions, I played with a guy last night who used a shotgun and would miraculously pop out around a corner for a split second and shoot you in the face, over and over, and he knew exactly when you were coming. If he was low on health, he'd wait until you were just around the corner, even if you walked or crouched it was no use.

I played with another guy who jumped straight up out of nowhere and shot me in the head.

I just quit when people are playing like that and find a new server. There are servers that have enough admins that all the cheaters are banned.