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
There are several Apple serial number formats and this tool supports them all from the 1980s right up to current day. The classic Mac serial number format decoding for 80s and 90s Macs is still in development but a date should be returned at the very least. You can link direct to the results (for example in eBay listings) by copying the URL and pasting that into your listing, post or email.
Apple serial numbers are between 10 and 15 characters long (letters and numbers) and do not contain any spaces, dots, hyphens or slashes. If your code has a slash or is shorter (perhaps starting with an A) that is the Apple model number and can be decoded using my Apple model number decoder. For details of how this tools works and how it differs from other Apple tools out there see the how it works page.