Using Internet Apple Hardware Test
One of the new features that appeared with Macs that shipped with Lion was Apple’s Internet Recovery. If you encounter a situation in which you cannot start from the Recovery HD, such as where the internal hard drive has failed or when you’ve installed a new disk without an OS on it, Mac models that were released after July 2011 can use Internet Recovery. Internet Recovery lets you start your Mac directly from Apple’s servers using a NetBoot-like process and gives you the same functionality as Recovery HD does.
Apple also made Apple Hardware Test (AHT) available in a similar fashion. Apple now ships most Macs with a hidden AHT partition, which allows you to hold down the D key at startup to boot your Mac using the AHT partition. From there, you can use the available tool to (hopefully) diagnose hardware problems on your Mac. However, if the AHT partition is not available and you have an internet connection, you can use Internet Apple Hardware Test. See below the jump for the details.
Update – May 26: The AHT partition is actually mounted from a disk image stored in the factory-supplied Recovery HD partition. If the factory-supplied Recovery HD partition is removed, or replaced by another Recovery HD (either by using the Mac App Store’s Lion installer or by other means), the AHT disk image is gone and there is no way I’m aware of to bring it back or rebuild it.
To use Internet Apple Hardware Test, start up your Mac and hold down both the option and D keys on your keyboard.
You should see a gray screen with an animated globe appear. It should say something like “Starting Internet Recovery. This may take a while.” Depending on your connection speed, it may also switch to a countdown clock to show you how long until it’s fully booted.
Once booted from Internet Apple Hardware Test, it takes a minute or so for the Apple Hardware Test software to start up and inspect your hardware configuration. While this is taking place, the icon below should appear on your screen.
Once the boot process has completed, you’ll be asked to select your language.
Once your language has been selected, the Apple Hardware Test console appears.
You can choose which sort of test or tests to perform:
To perform all of the basic tests, click the Test button with the mouse or press the T key or the Return key on your keyboard. To perform a more thorough diagnostic test, select the “Perform extended testing” checkbox under the Test button before you click the Test button.
Your test results will appear in the window in the bottom-right of the console. To exit Internet Apple Hardware Test, click the Restart or Shut Down buttons at the bottom of the window.
For more information, see Apple’s KBase article on using Apple Hardware Test: http://support.apple.com/kb/HT1509