How do I run Apple Hardware Test on a 2012 MacBook Air with 10.8.4?

Using Apple Hardware Test on computers with OS X 10.7 or later

Mac NoteBook and Desktop computers shipping with OS X 10.7 or later have a partition on the hard drive or flash storage that contains AHT. To start AHT on these computers please follow these instructions:

1. Press the power button to turn on your computer.
2. Press and hold the D key before the gray startup screen appears.
3. It takes a minute or so for AHT to start up and inspect your hardware configuration. While this is taking place, an icon appears on the screen:
   
4. When the process is complete, select your language and click the right arrow. If you aren't using a mouse, you can use the up and down arrows to select a language and then press the Return key.
5. The AHT console appears. You can choose which sort of test or tests to perform:
   • To perform all of the basic tests, click the Test button or press the "T" key or the Return key.
   • To perform a more thorough diagnostic test, select the "Perform extended testing" checkbox under the Test button before you click the Test button.
Note: Your test results will appear in the window in the bottom-right of the console.
6. To exit AHT, click Restart or Shut Down at the bottom of the window.

Some Macintosh computers that shipped with OS X Lion and later support the use of Apple Hardware Test over the Internet. These computers will start up to an Internet-based version of AHT if the hard drive does not contain AHT. An Internet-enabled connection via Ethernet or Wi-Fi is required to use this feature. Internet-based AHT functions the same as AHT on the hard drive or flash storage outlined above.

The Mac NoteBook and Desktop computers listed in Computers that can be upgraded to use OS X Internet Recovery will require a firmware in order to support the Internet version of AHT. Without this update you may see the behavior discussed inthis article.

http://support.apple.com/kb/HT1509