Serial number for Apple Watch

Look on the original box or sales documents if you still have them. Look in the Watch app on your iPhone, My Watch/General/About. Look in Devices in your Apple ID, I think it is also listed there.

If your Apple Watch is lost or stolen - Apple Support

If you have the original packaging it will be on there. I don't think it will do you much good. If you are thinking Apple will help track it....they won't.