There is an acid test. Simply take the watch you're considering buying to a dealer of the particular brand and ask to have it serviced. If it's fake they won't touch it. I would always do this before buying any high end watch.
Originally Posted by Mustafa Umut Sarac
Rolex movements (with the exception of the Cosmograph Daytona) have been made in Asian sweat shops for years. Only 10% or so of the movements are good enough to be certified as Swiss Chronometers. The rest are sent back to China where many end up in fakes. They run just as well as certified ones.