|To test the PSU, unplug the main ATX plug from the board, bend a paper clip (or piece of wire) into a U-shape, put one end in the pin to the green wire & the other end to any black wire. This will bypass the power switch on the case & allow the PSU to power up as soon as it's plugged in or switched on. You can then test the voltages with your multimeter. Yellow = +12v, Red = +5v, Orange = +3.3v. The tolerance is 5% plus or minus.|
However, amperage is what's important & there's no easy way to check for it. For example, just because your +12v may test within spec, it doesn't mean that it can supply +12v at say, 15A. The equipment for load testing costs 1000's of dollars.