On Rolex watches manufactured before 2005, the serial number is located underneath the end piece between the lugs at the six o'clock position. Rolex watches manufactured in 2005 or later display the serial number inside the inner rim of the case, also known as the rehaut.
To view the serial number on a pre-2005 Rolex, insert a push pin into the band pin located at the top of the lug. Repeat on the other side. Take off the end piece to expose the serial number. On a 2005 or post-2005 Rolex, go to the six o'clock position to see the serial number engraved in the rehaut. It is surrounded by the word "ROLEX" which is repeatedly engraved around the circumference of the dial.