From your device home screen, hit the Settings App > General > About > Serial Number
How to find the serial number for your Apple Watch
Check the case of Apple Watch
Find Serial Number Macbook Mac iMac:
If your computer is turned off or won’t turn on, you can flip your Mac over and find the serial number physically printed on the hardware. Look for the text beginning with “Designed by Apple in California,” and then look at the lowest line of writing, where you’ll find the serial number.