How can you tell if a Rolex watch is authentic or fake without going by the serial number?
The only real guaranty of receiving a genuine Rolex is if you purchase the time piece from a Rolex Authorized Dealer. If you plan on purchasing a Rolex privately, buy locally. Have the seller meet you, time piece in hand, at a Rolex Authorized Dealer that has a watch maker on site to verify authenticity.
Avoid buying a Rolex through the internet, especially through auction websites.
You can locate a Rolex Authorized Dealer through the official Rolex website. Keep in mind that not all Rolex Authorized Dealers are created equal. Do your research by searching the various online watch forums and ask for local recommendations.
Most modern Rolex contain automatic movements but contrary to popular belief, they do tick -- 4 to 8 times per second, which gives the appearance of a sweeping motion. Rolex has offered a model with a quartz movement (Oysterquartz) but ceased production after 2001. So sweeping hands versus ticking is not always a clear indication of authenticity, unless the watch is ticking in one second intervals and it is not a Rolex 5035/5055 movement Oysterquartz.
There are a number of ways to help you identify an authentic Rolex from a counterfeit. For more information, an excellent resource is QualityTyme.net. The site is owned and operated by the author of "The Rolex Report."
A few of the most obvious identifiers is the case back and "cyclops" date magnification...
Rolex does not produce time pieces with exhibition (a.k.a. clear or "skeleton") case backs that allows you to view the movement. Unless you happen to run across one of two vintage 1930's manual wind movement Rolex, it is more likely than not, a fake.
The "cyclops" date magnification should be 2.5 times. Most counterfeits are smaller (1.5 times.)
Feel free to PM me if you have any questions.