how do i find the serial number of a stolen macbook air

There are many way to interpret your question. Please provide more details.

For example:

"How do I find the serial number of my MacBook Air that was stolen?"

If you bought it directly from Apple, look on your original receipt, or on the packaging, if you still have it.

"I have recently come into possession of a stolen MacBook Air. How do I find its serial number to report it?"

Check the underside of the MacBook Air - the serial number is printed in 4pt grey text alongside the Model No., FCC label, etc.(get a magnifying glass to read it).

"I have received a MacBook Air that I think might be stolen. How do I check if its serial number is listed as stolen?"

There is no universal, public database of stolen devices. You can report the serial number to Apple or to your local police, but there is no direct way for you to trace it back to its original owner.

Or anything number of other ways of interpreting your request.

doninpostfalls Said:

"how do i find the serial number of a stolen macbook air"

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Finding the Serial Number:

If this is your Mac, then use this link: Check your Apple ID Device List to See where you're Signed in - Apple Support