When you switch on the watch for the first time, you will see the "not yet activated" message. The serial number and the watch phone number or SIM ID of the watch should be displayed on this screen.
If the SIM ID is missing, you can click on the link at the bottom of the screen that says "Register Later". Now you should see a screen which displays the time. You can return to the home screen by pressing and holding the red button for a few seconds. Now the SIM ID should be displayed.