Apple serial numbers are normally printed on the back or bottom of the device, a few other places they can be found are :-
For an Apple Mac within "About this Mac"
For an Apple iPhone/iPad on the about screen (Settings > General > About)
For a modern Apple iPhone it may also be printed on the SIM tray
For Apple AirPods it's on the underside of the lid, the packaging or when connected to it it's on the about screen (Settings>General>About>AirPods)
For an Apple HomePod it will be on the bottom of the device
There are a few Apple serial number formats and this tool supports them all from the 80s to current day. Classic Mac serial number decoding is a work in progress but should return a date at least. You can link direct to results (e.g. in eBay listings) by copying the URL and pasting it into your listing, post or email.
Apple serial numbers are 10 to 15 characters long (letters and numbers) and don't contain any spaces, dots, hyphens or slashes. If yours has a slash or is short (perhaps starting with an A) that is the Apple model number and you should use my Apple model number decoder instead. For details of how this tool works and how it differs from other Apple tools see the how it works page.