How can I perform an Apple Hardware Test without my original install media?
I've never been able to figure out why this function seems so unreliable - holding D for diagnostics doesn't seem to work for me in practice. I wish I had a better explanation than "many models don't have this and even the ones that do, this isn't something I can count on."

I have come to rely on AHT that are available for authorized Apple technicians to download so you might be able to get help from one closer than an Apple Store - http://www.apple.com/buy/locator/service/

Apple will usually run a very good test for free as past of a genius bar appointment, and both Apple and Authorized Service will usually check a mac in for diagnostics under coverage for no charge. Out of warranty, expect to pay $39 to $85 for a diagnostic.

Lastly, you can order replacement media if yours are misplaced from Apple. 800-APL-CARE