Can I tell if a found Macbook Air is lost or stolen?
No, it is impossible to tell if a found Macbook Air is lost or stolen via physical examination of the device or its operating system.


Computers of the Apple MacBook Air line are not equipped with Kensington security slot which might indicate theft if damaged.
Mac OS X or any other contemporary operating system that can be installed on MacBook Air device does not have any property that changes at the moment of theft.


Apple Inc. published the article "Report a lost or stolen Apple product" on their official support pages with clear instructions for users of Apple products which got lost or stolen:


  If you have lost or found an Apple product, please contact your local law enforcement agency to report it.


Quoted from the version last modified on Mar 18, 2016

Hence the only official way to make sure if a particular unit was stolen is to contact appropriate local law enforcement agency.