As John McClane begins taking down the bad guys in the building, he notices this fact when searching the bodies, so when he sees Hans Gruber’s watch, when handing him a cigarette, he knows he’s dealing with another one.
Well, it goes back to an earlier scene that ended up being cut from the film.
What does the watch have to do with anything?
Screenwriter Steve E. de Souza explained at a 30th-anniversary screening of The Running Man, covered by Slashfilm, that there was originally a scene at the beginning of the film where Hans Gruber and his team all synchronize their watches, and the audience sees that every one of them is wearing the exact same watch.