Apple iMac (21.5 and 27-inch, Late 2009) - Manual (Page 60)

60 Chapter 4 Problem, Meet Solution
Locating Your Product Serial Number
Use any of these methods to find the serial number of your iMac:
 Choose Apple () from the menu bar and then choose About This Mac. 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.
 Click the Finder icon and open /Applications/Utilities/System Profiler. Click Hardware
in the Contents pane.
 Look on the bottom of the iMac stand.