Apple Mac mini (Mac OS X Server, Late 2009) - Manual (Page 50)

50 Chapter 3 Problem, Meet Solution
Locating Your Product Serial Number
Use any of these methods to find your Mac mini serial number:
 Choose Apple () from the menu bar and then choose About This Mac. Click the
version number beneath the words “Mac OS X Server” to cycle through the Mac OS X
Server version number, the build version, and the serial number.
 Open System Profiler (in /Applications/Utilities/) and click Hardware.
 Look on the bottom of your Mac mini.