How to use a multimeter to test Power supply?

I have seen many G5 iMacs fail due to both PSU issues as well as capacitor problems. There are two types of caps on the logic board and (like most caps) when they die, they bulge and leak electrolyte. Before checking the PSU, make sure that all of the capacitors look good. They /are/ replaceable if they're bad and you're handy with a soldering iron.

Per Step 12 of this Apple Knowledge Base article: http://support.apple.com/kb/HT2173,

Your power supply has failed (and this is unfortunately a common issue, due to counterfeit capacitors sourced by Apple and many other major manufacturers at the time). No multimeter necessary.

The power supply is available here at iFixit, and you've already done the hardest part!

you will need to connect the black lead of the volt meter to the ground cable pin (Ususaly Black) and one to the colored lead you want to test, but this is very hard to do, because the power supply has to be tricked into thinking it's on, so you need a 470OHM resister across the standby lead (usually green) and to a ground lead, than test the voltages, the yellow should be around 12V, the Orange around 3.3V, and the red around 5V, the blue will be -12V, but the colors may be different, but that is the standard DC color coding.

Another power supply component that recently failed on me is the power mosfets located on the bottom of the PS circuit card. There are about 8 ea. of them , 2 for each voltage from 3.3 to 12vdc. Mine had a part number of PHP101NQ. I replaced 3 on my power supply. Found a good sub with a little higher amp rating at Newark.com

For checking power supply first you have to set your multimeter on dc volts, now check output terminals of power supply whether it’s giving proper voltage level or not.