Let's say you buy a pre-owned or vintage watch and want some more information on when it was made, what it might be worth, etc. The watch's serial number can, in many cases, help you track down this information in conjunction with online or company-owned databases. But just where is this number located? Unfortunately, it sort of depends on the watch, but here are some tips to tracking it down:
If you have paperwork from the watch, the serial and reference numbers should be there
You may be able to save yourself some detective work if you have your watch's papers, such as the warranty, receipt, etc. The model number may be referred to as the "style number" or "reference number," while the serial number may be listed without any sort of qualifier. When it doubt about which is which, see the note below regarding online databases.
See if there's a free online database of serial numbers from the company
Often, you can find online serial number databases that will help you narrow down the age of your watch to the year it was manufactured, or thereabouts. This won't necessarily tell you things like the country to which the watch was delivered — for something like that, you may need to pay to receive an "extract" from the company's archives, which can cost you a few hundred dollars. (Longines, uniquely, offers this service for free for any of its watches — you simply provide them the numbers off your watch and some images and then wait for an answer.) But looking these tables up ahead of time may help you differentiate between a watch's serial number and reference number.
Check the back of the watch
Depending on the brand, the era in which it was made, etc, the watch's serial number may be present on the case back — this is especially true of many more modern timepieces. However, you'll need to learn to differentiate between a serial number, which identifies an individual timepiece, and a reference number, which is the model number. If in doubt, plug in a search for each number along with the name of the watch company and see what comes up.
If the watch is a Rolex, check between the lugs
Rolex is unique in that it parks its model and serial numbers between the watch lugs. Remove your bracelet or strap and these should — hopefully — be visible. Model numbers are generally four, five or six digits long, whereas serial numbers can be up to eight digits long. Again, when it doubt, plug both numbers into Google with the word "Rolex," and you should be able to tell which is a model number and which is a serial depending on the search results.
If the watch has a transparent case back, look at the movement
Is your watch a modern model with a sapphire case back, through which you can see the movement? It'll probably be difficult to discern the serial numbers — if they're engraved on the movement — from the case back, but it's worth taking a look, especially if you have a jeweler's loupe on hand.
Open up the case back
It's best to have a jeweler do this so that you don't scratch the case back, but this is something you'll have to do (or have done) in the case of many vintage watches. Vintages Longines watches, for instance, feature both the serial and model numbers (as well as movement numbers — more on that in a moment) within the watch case. The serial number might have the case serial and model numbers, while the movement itself might have a separate number.
The case and movement might have separate serial numbers
Keep in mind that the movement on a vintage watch might be signed with its own traceable serial, while the case might have a separate serial. In some cases you might need to provide both these numbers, as well as images of the watch, to a company for tracing. (In an instance in which a watch company contracted with different case manufacturers, having separate case and movement serial numbers allowed them to carefully track production from different companies.)