How to find the serial number on macbook

See the following from Identify your MacBook Pro model - Apple Support

If you don’t have your Mac or it doesn’t start up, use one of these solutions instead:

• Find the serial number printed on the underside of your Mac, near the regulatory markings. It’s also on the original packaging, next to a barcode label. You can then enter that serial number on the Check Coverage page to find your model.
• The original packaging might also show an Apple part number, such as MLH12xx/A (“xx” is a variable that differs by country or region). You can match the Apple part number to one in the list below to find your model.