If you can't read the engraved text on the watch or don't want to remove the band, the model number can be found in iOS settings.

Open the Watch app on your iPhone and select the "My Watch" tab.

Select General > About and then scroll to the "Model" field. A number will be displayed as a combination of letters and numbers starting with “M”.

Tap the "Model" field. You should see the model number which starts with “A” (e.g. A2473).