Where to find the serial number of my previous Apple Watch?

Hello!

You can find serial numbers for all devices signed in to iCloud by going to Settings > Profile (your name at the top) > Scrolling to your devices > Click the Watch you need. If it doesn't show up, this is because you have removed the device from your account. If this is the case, you might be able to find it on the proof of purchase, or if you have ever had an Genius Bar session or any other Apple session, it should be in the email that was sent.

Hope this helps!