Make Apple Hardware Test work on Mid 2009 MBP with OS X Mavericks
it is d you have to hold down to run the Harware Test from the Second DVD that cames with your Mac when you Buy it. I don't know how it is on the Macs that don't have a Disc-Slot. Maybe Apple has added the Hardware Test to the Recovery Partition.

Keep in Mind what do you do if the Check brings up a Failure? - And all the Time you maybe need to figure out what exactly is the Failure from a cryptic Error code!! - I highly recommend you ro bring your Mac to a local apple retail store so they can check it for you it will cost something but still less then the time you will be spending to do it yourself.