Below is a couple of responses for diagnosing power supplies that I found awhile ago. I'm not the author of these responses so I can not take credit for them (nor do I remember who was)and I have not tried them but I did save them for future problems.
<< To verify that the culprit is really the power supply, first make sure it's plugged into the wall, and that there's power to the power supply (plug in, turn on switch on power supply if applicable). Then unplug the power supply from the motherboard by disconnecting the ATX power block. Holding the power block in your hands, you should see a -gray- (or grey, if you prefer) wire. this wire is probably going to be thinner than the rest. That's because it doesn't supply much current/voltage. It is a signal wire. Use a piece of wire, or anything, and short this gray wire with a ground wire (black). A working power supply will turn on. You can tell by the spinning fan inside. Or, if you suspect that the fan inside may be broken, you can measure the voltage of the RED, YELLOW, etc. wires... anything but GRAY, BLACK, and GREEN should have some positive or negative voltage. Don't worry about killing yourself, the maximum electrical potential is about +/- 12V. Anyways.. if the power supply DOES NOT turn on. Then just replace that.. it's cheap.. >>
<< you suspect the power supply, remove the PSU from the computer and carefully use a paperclip to connect the green wire (on the plug that usually goes right to the mobo) to any of the FOUR black wires. this will force the PSU to turn on, and will tell you whether the power supply is any good. >>
Hope this helps.