Generally when folks are really hacking it's fairly obvious. I've seen people enter a server and knife everyone in it in seconds including those inside of vehicles. Or killing people from their spawn base at ours with a shotgun or pistol a second after the round starts. Generally when people use the aiming bot parts of these hacks on the death cam their character will appear VERY twitchy as the hack targets people.
Trouble is the "smart" hackers don't use all of those. They can easily use the no recoil option of most of the hacks to give them an edge and that's much harder to detect for a player who just got killed. Same goes with the options that allow you to see through walls like ...http://www.youtube.com/watch?feature=player_embedded&v=dAOvNRA3G7c#!
that. You can't really tell if the person is using that or if you were just spotted by someone and they knew you were there. =P I worry more about the number of people using those that you just can't tell then the folks who teleport an entire team onto a stick of C4 and detonate it or knife people in tanks. They are usually dealt with fairly quickly.
Every night if I play for 3-4 hours across a half dozen servers I will usually see at least 1 person blatantly hacking. But I daresay the actual number is a good bit higher and the people using them just aren't as obvious.