Acting on a tip off, the police raid a flat to arrest a suspected murderer. They don't know what he looks like but they know his name and that he is inside the flat. The police burst in and discover a bricklayer, a taxi driver, an electrician and a fireman playing cards. Without any hesitation or communication they immediately arrest the fireman.
How do they know they've got the suspected murderer?
(I do know the answer to this, but thought I'd share it)