Trivia: In the book, Aaron does not have a stutter - the stutter was introduced for the film because when Edward Norton auditioned for the part, he did the lines with a stutter and it won him the part.
Trivia: The part of Aaron Stampler was originally written for Leo Dicaprio. He turned the role down and it was given to Edward Norton.
Answer: They knew he was a street kid and looked where the homeless and beggars hang out (under a bridge) they recognised him by his bandaged ear, from where the crucifix earring was ripped.