Testing Your PSU Rails - EXTREME Overclocking Forums
You set the multimeter to the lowest DC voltage it has, then, while the computer is on you place the black lead into a connector with a black wire.
The red lead will read different rails depending on the color of the wire. If the wire is red its 12V, orange is 5V and green is 3.3 (I think).
This should be done both on load and idle, idle can just be letting the comp sit at desktop for a minutes with nothing running.
Load can be on prime 95 or vantage/3dmark
Rails have to within 5% of themselves, so for example the 12v rail has to be within 11.4-12.6 volts.