How to test a power supply?
Unplug the power supply from any of the components inside the computer (or just remove it from the computer completely).
USE CAUTION HERE (Though you'd only be shocked with a max of 24 volts)
- Plug the power supply into the wall.
- Find the big 24-ish pin connector that connects to the motherboard.
- Connect the GREEN wire with the adjacent BLACK wire.
(If you're colorblind, the green wire sits between three black wires on one end and one more on the other)
- The power supply's fan should start up. If it doesn't then it's dead.
- If the fan starts up, then it could be the motherboard that's dead. You can use a multimeter to check if there is power output from the power supply.
Most well-stocked geek-stores sell a "power-supply tester" that has all the appropiate connectors to plug each part of your PSU into, with spiffy LEDs indicating status of the various rails, connectors for IDE/SATA/floppy power cables, etc. They run ~$20 US.
You could try using a multimeter to check the +/-5V and +/-12V rails. Due to the switching nature of the power supply, you'll want to put a resistor in series with your probes while measuring.