If you have no trouble finding the UDID of your iPhone / iPod touch / iPad with iTunes, the method is somewhat different for the Apple Watch. For the latter, you need to first download Xcode if you want to recover its UDID.
Our colleague IDB explains on its website how it is possible to do it in a short tutorial. Nothing complicated, just bring along Xcode and follow the four steps below.
Before you begin, you must connect the Apple Watch on your Mac, which itself must be paired with your iPhone.
Step 1: Download Xcode from the Mac App Store
Step 2: Launch Xcode
Step 3: Click “Windows”-> “Devices” in the menu bar
Step 4: Click your iPhone in the sidebar “Devices”, and you should see your information about Apple Watch under Information. Your UDID will be listed next to the field “Identify”. To get your UDID, simply right-click to copy it to your clipboard.