To find your Rolex's serial number remove the strap at the 6 o'clock position and it will be engraved on the case between the lugs.
It will be 5-7 digits and may start with a letter. It will NOT have letters in the middle, if your serial number has letters in the middle it may be a model number or they may just be misread badly stamped numbers. The Rolex model reference can be found between the lugs on the opposite side of the watch, that is between 4 and 6 digits and is numbers only. This model reference will decode to tell you the model of watch, the material it is made from and the style of bezel it should have. You can also look up the date from the Rolex movement/caliber number etched directly on the movement using my Rolex movement database or date the strap/clasp using my Rolex clasp date decoder.
This can't validate the authenticity of a Rolex watch, but it checks the serial number to find the year of manufacture and decodes the model reference. If the Rolex type wasn't being manufactured during the year suggested by the serial number or if the model reference suggests something different to what you have then it suggests further investigation is required to check it is a genuine Rolex. Please note, Rolex watches made after 2010 no longer have sequential serial numbers and cannot be dated this way.