Rolex model or reference numbers are listed on the certificates Rolex issues, but its also available on the watch case itself. You will have to remove the bracelet to find it. The reference/model number is engraved between the lugs on the 12 oclock side where the bracelet meets the watch case as pictured below.

Where is Rolex Datejust reference number?

A Rolex reference number engraved between the lugs right above the 12 oclock location on the side of the case.

Where is the reference number on a watch?

In some cases, the reference number is engraved on the case. For example, reference numbers for Rolexes sit between the lugs at 12 oclock. To see it, you have to remove the band. Other manufacturers may place it on the case back or, in rare cases, on the dial itself.