When you purchased your Macintosh, the box should have included a CD or DVD with an application called Apple Hardware Test. Depending on when you bought your computer, this could be an independent disc, or it may be included on the Mac OS X Install Disc. (Look for tiny lettering on the disc that says "To use Apple Hardware Test, hold down the Option key as the computer starts up," or words to that effect.) Find this disc now. (I'll wait while you root through your attic or basement to find it hidden in the bottom of a box somewhere.)
Back already? Super. You have in your hands a very special program. Apple Hardware Test can run only when you start up from the CD or DVD it came on; don't bother trying to copy it to your hard disk. This program performs a series of diagnostic tests on your Mac's hardware, including the AirPort card, logic board, hard drive, RAM, modem, and video RAM. It doesn't repair anything, and it doesn't look for problems such as directory errors that are the province of Disk Utility (described in the next section). But it can identify subtle hardware defects that could later lead to serious problems. Whether your Mac is fresh out of the box or years old, you owe it to yourself to make sure its major components are in good health, and this is the easiest (and cheapest) way to do so.
Apple Hardware Test isn't the only tool that can check your RAM. Among the other utilities that can do this are TechTool Pro (www.micromat.com; $98) and Rember (www.kelleycomputing.net/rember/; free). I've personally had bad RAM that Apple Hardware Test could identify while these others could not, whereas other Take Control authors have had the opposite experience. Your mileage may vary!
To run Apple Hardware Test, follow these steps:
I recommend running the test again after installing RAM or any other new hardware inside your computer, or if you begin to have inexplicable problems that ordinary disk utilities do not solve.