Apple MacBook Air (Late 2008), (Mid 2009) - Manual (Page 55)

Chapter 3 Problem, Meet Solution 55
Locating Your Product Serial Number
Use one of these methods to find your computer’s serial number:
 Turn your MacBook Air over. The serial number is etched into the case, near the
hinge.
 Choose Apple () > About This Mac, and then click the version number beneath the
words “Mac OS X” to cycle through the Mac OS X version number, the build version,
and the serial number.
 Open System Profiler (in /Applications/Utilities/) and click Hardware.
Serial number