From Dori Reuscher,
Your Guide to Computer Certification.
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Troubleshooting

If you have an issue that could be related to the computer's power supply, first make sure that there is electricity coming from the AC outlet. Next, measure the AC voltage with a multimeter.

Multimeter
The multimeter, or Volt-ohm meter (VOM) measures both continuity - the existence of a connection, and resistance - the amount the wire resists the flow of current, measured in ohms. A good fuse measures a resistence of zero ohms, while a bad fuse registers a resistence of infinity ohms. A typical current from the wall outlet will be 110v or 115v.
After testing the outlet, you can test the motherboard connectors. A good DC voltage is between 11 and 13 volts. External devices can also cause power issues. Disconnect all devices when testing the motherboard to isolate this issue.

Basic Terms

DC Power - The primary purpose of your computer's power supply is to convert AC power from the oulet to DC power for the motherboard. DC power flows in one direction, from "-" to "+".

AT & ATX Connectors - AT connectors, P8 and P9, are older and require that you keep the black ground wires next to each other to avoid damage when plugging them into the motherboard. The ATX form factor elimitates the risk of damage by using a "keyed" P1 connector. The ATX also includes a 3.3 volt wire in addition to the standard 5 and 12 volt wires.

Power Switch - There are two types of AT power switch, plunger and rocker. The AT switch turns a system on or off, while the ATX switch uses soft power to handle power management issues such as hibernation and sleep modes. The ATX power supply never completely turns off and continuously supplies 5v to the motherboard.