There was a time when one had to take a trip to New York City to cross paths with a fake Rolex for $25. It was practically a tourist attraction: See the Statue of Liberty, catch a Yankee game, get offered a fake Rolex from a petty criminal. It used to be exciting to buy one when they were $25--the purchase was half the fun and it made for great cocktail party conversation. The problem is now they're everywhere: Online auctions, pawn shops, vintage second-hand shops and good charlatans are scamming people for real thousand-dollar Rolex prices.
Take a look at the second hand. Is it ticking or is it sweeping? If it's ticking, that's your fake. A Rolex second hand sweeps around in one continuous motion. But beware, as of 2005, the good fakes have caught up to this technology.
Look for the small engraved crown at the 6 o'clock mark on the crystal. Most fakes will have some sort of large, obvious version of a crown, but this mark on a real Rolex is almost impossible to spot with the naked eye.
Look for the word "Rolex" and the watch's serial number engraved at 6 o'clock under the crystal on the area between the crystal and the dial. While many fake Rolex watches post-2005 have this feature, they're usually printed, not engraved.
Most new Rolex models have an added model number engraved at the end of the watch end-link. This engraving includes the band model and two tiny Rolex crowns. Your good fakes will have something similar, but not all three indicators.
If you're being sold an authentic Rolex (in good condition and in perfect working order) for anything less than $500, become suspicious. If it's in nice shape, it doesn't really lose a lot of appreciation, and in some cases, it increases in value.