Use the other teams perks against them.

Scrambler, when an enemy is nearby your map will scramble, the closer they are the more it scrambles. Use how much its scrambled to find them, you can easily kill them before they kill you.

SitRep, if you have claymores lay them, if someone is smart enough to use sitrep to try to reach you you'll notice they will always blow up your claymore before trying to kill you, when they blow it up you know, use them blowing up your claymore with sitrep against them.

Lightweigt, if you see someone running very fast you know that have this on, if you happen to have stopping power (maybe FMJ also) on they will not stand a chance in a full out gun battle, keep your distance its almost guaranteed they have marathon and or commando for close range kills and probably an SMG so mid range gun fight easy win.

Cold blooded, this works well even if they don't have cold blooded but its more likely if you find that they do. If you see someone, probably a sniper in one spot and you happen to notice they have cold blooded (or not but if they do its more likely this will happen) kill them, go to where they were and lay some claymores, theres a chance at some point they will go back to that spot and you will get the claymore kill, and it tells you they will probably go back again if you wish to repeat this.

One man army, as much as i hate to say it my favorite perk has a flaw. If you notice someone is playing and they seem to always be using different weapons and haven't died much, or you see claymores and tactical insertion, or happen to watch the kill cam and notice they have it, most likely they will use it right after they kill you, if their smart they will want the ammo back and if you set off their claymore they will want a new one, use tactical insertion near by over and over eventually you can break through their defenses and sometimes catch them changing class unable to fight back. Although thats the perks flaw i don't suggest doing this alone, this could easily cost you enough lives to give them a high kill streak if they know what they are doing.

These are not something to just look for ever, if you just happen to notice this in a match, you may go 20 matches or more without seeing any of these but if you just happen to see one now you know what to do.