Since the Apple Watch has no summary page in iTunes, you’ll need to use Xcode in order to find the Apple Watch UDID. In this post, we’ll show you how to do just that.
Step 1: Download Xcode from the Mac App Store
Step 2: Launch Xcode
Step 3: Click Window → Devices in the menu bar
Step 4: Click on your iPhone in the Devices side bar, and you should see your Apple Watch information under the Paired Watch Information heading. Your UDID will be listed next to the Identifier field. You can simply right-click on the UDID to copy it to your clipboard.