Check the Settings menu
On your Apple Watch, open the Settings app, tap General > About, then scroll down.
On your iPhone, open the Apple Watch App, go to the My Watch tab, then tap General > About.
Or check the case of your Apple Watch
Learn how to identify your Apple Watch.
On Apple Watch (1st generation), check the back of your watch for the engraved serial number.
On Apple Watch Series 1 and Apple Watch Series 2, including Apple Watch Hermès and Apple Watch Nike+, remove the band from your watch, then check the band slot.