One of the easiest ways to spot a fake Rolex
(or any other high quality watch) is to look at the movement of the second hand.
If it sweeps smoothly across the dial, then it's been manufactured to a high standard.
If it 'ticks' across the dial, it hasn't.
This is mostly due to the internals of the watch. In order to make them run smoothly, the contact points between components are, if memory serves, capped with diamond to reduce friction.
Of course, the other two ways to spot a fake Rolex: It's being sold for $20, and it's spelled 'Roelx' on the dial.