If you get into a game where there are people around that can tell, they will post your name up on Xbox's forum to ban you and report you. Its easy to tell when someone is lag switching. Example:

I was the last alive in S&D and the second I planted the bomb, [sarcasm] just all of a sudden [/sarcasm] I started lagging and the guy on the other team got a perfect headshot. When the person has a shitty connection, they play the same as you, but if they are able to get a headshot like that fucker, then you know its a lag switch.

BTW, I make sure every person who use lag switches on me are sure to get banned within a day.