I lifted the following from a posting on Google Groups. It describes a method for checking the voltage on each of the pins of an ATX power supply.
To test an ATX power supply with little risk of damaging the PC, you will need some jumper wires, a car turn signal bulb (1156 or 3156), a voltmeter, and the picture of the ATX power supply pinouts at http://www.technick.net. Note that the wire colors you have may not be the same as on that Web page, but the pin numbers should be.
Completely disconnect the power supply from the PC and remove it. With the supply unplugged from the wall, install a jumper between the "turn on" pin (14) and any of the ground pins. Wire the tail-light bulb to one of the +5v pins (4, 6, 19, 20) wires and any of the ground pins -- this provides a load so the supply will come on. Turn on the supply (plug in the supply?) - the fan should spin and the bulb should glow dimly. Now use the voltmeter to measure between one of the ground pins and every other pin, writing down what voltage you get at each pin. A digital voltmeter is best, but an analog one will work - just be careful not to "slam" the meter on negative voltages.
When you have everything recorded, turn the supply off, unplug it from the wall, and remove all jumper wires and the light bulb.