Turn off the PC, but do not unplug it, open the system unit. Set the multimeter to read DC volts in the next range higher than 12 volts. Locate a power connector similar to the hard drive, or floppy drive connector that is unused and turn on the PC. You can also unplug a drive connector and use it as well. Turn on the PC and insert the BLACK probe into power connector on one of the BLACK wires. Touch the RED probe to the YELLOW wire on the power connector.
Figures below illustrate the procedure for checking the power supply in a typical ATX power supply.
The multimeter reading should be +12 volts. Now touch the RED probe to the RED wire and the reading should be +5 volts. If no readings or different readings occurred, you, will have to replace the power supply. If the readings were correct, you should check the connector at the motherboard.
Refer to the power supply connector/voltage details shown in the figure below and check the corresponding voltages.
Check the power going to the Motherboard connections by inserting the multimeter probes into the connectors as shown in the figure below.
DO NOT remove the power supply from the system unit case when performing these tests Be sure to remove any and all electrical static build-up from your clothes and body BEFORE touching any parts inside the system unit.