How do I find serial numbers for all my Apple devices?

You can look on the bottom of the MacBook.

Or if you have logged in with your Apple Account on the MacBook, the serial number can be found in the Devices List on any other device logged in with the same Apple ID.

Or if you don't mind waking it up, you can find it in the  Menu ➜ About This Mac.

click here ➜ Find your Mac model name and serial number - Apple Support

To find your Serial Numbers you can do it these ways go to Check Your Service and Support Coverage - Apple Support and log in to your Apple Account and you should see your device if they are linked to your Apple Account you can also check on the Original Packaging at the back you can also check at the  Menu by clicking it then clicking About This Mac then your Serial Number should appear there if you want to know more check out this article from Apple Support! Find your Mac model name and serial number - Apple Support

McWagon wrote:

thanks, the 'Apple Account' is the key. Unfortunately, my mac book air, Applesauce, has been on an extended nap, and no amount of patience with the recharger is bringing her back to life. My tech-savvy daughter says, "It's time to replace the battery." With the Serial #, I can proceed. MNI thx.

As already said above, the serial number is printed on the bottom of the computer.

McWagon wrote:

the serial number is visible only thru the Magnifier app on the iPhone. But it's a 3-handed job to transcribe it to text. Again, thanks for the 'Apple Account' note.

Indeed it's a wee bit small.