Somebody who is truly 1337 will be told by others, the actions of a true 1337 player will let everyone know they're 1337, so they will not have to tell you, anybody who feels the need to say they're 1337 clearly removes any kind of 1337ness they may have held, they infact become a n00b.
1337 players will pwn (own) everybody not just n00bs, you really don't have to be 1337 to pwn a n00b.
Ways to identify 1337 players:
People who pwn (own) everybody else in the game
People who are good at what they do
People who don't claim to be 1337
Players that pwn n00bs in incredibly skilled or funny ways