In case you don't already know, a smurf is a above average or skilled player that cloaks himself/herself with a new account and plays skirmish games against anyone in the lobby.
Their goal?
A) Get skirmish rating with perfect games (aka 2000 with 15 games or so, etc.)
B) Play with someone they dislike
C) Glitch with the fake account (usually doesn't work.)
The easiest way to find out who the smurf is to just observe how they type. Most people can't stay in an alternate persona for too long, so even one thing makes them obvious.
Another is to just find their IP. Unless you have the tools do this, you should try find different ways. Watch how they play in observe, and if they don't allow observers then it's obviously a smurf. With the community so slim now-a-days it's even easier to detect smurfs, so the point of smurfing is becoming irrelevant in 2012.
Edition #1
Signed
Deus