Collection of Apple Hardware Tests.
Early models can be tested by creating a bootable CD.
I'm not sure at what point USB sticks work better.
To create a bootable CD either use Disk Utility or right click on the DMG and use the "Burn Disc Image to Disk" opo-up option.
For info on how to create USB bootable test sticks (and more possible test/machine matches).
https://github.com/upekkha/AppleHardwareTest