I use CPUID HWMonitor and utility that comes with MB, but other free ware is avail.
First you need to verify software works HDMonitor incorrectly reports my +12 V as a - 12 V. If the values are way off then it is a invalid value. I prefer to verify my voltage with a DVM, then I KNOW if I can trust the software. With a DVM block lead to black wire on a Molex conector. Red lead to the colored wires on the Molex (red = +5V and Orange or yellow = +12V.
With the software runing and the computer at idle Note the voltages (HDMonitor shows current, Nin and max values. Now run Prime 95 (or furmark in a window) and note the decrease in voltage, the less drop the better. +12 Must be above 11.4 (I chuck my PSU if it drops below 11.6V). The +5 V must not go below 4.75V - max for +5 V is +5.25 (These are standards, but also Min/max for TTL logic chips). The max for the +12 v is +12.6 - But as long as not above 12.8 should be fine.
The Big diff between a good PSU and a Great PSU is in the drop of the +5/+12V going from Idle to Full load. This is also where you may first see signs of a PSU going bad.
As jsc indicated, need a o'scope to check of spikes, ripple and noise. You can use a DVM to check for ripple, if under 20/40 KHZ (switching Freq and 60 Hz for AC Line rejection) by placing the the DVM in AC mode. should be in the millivolt range.
Here are the ATX specs:
Supply [V] Tolerance Range (min. to max.) Ripple (p. to p. max.)
+5 VDC ±5% (±0.25 V) +4.75 V to +5.25 V 50 mV
−5 VDC ±10% (±0.50 V) –4.50 V to –5.50 V 50 mV
+12 VDC ±5% (±0.60 V) +11.40 V to +12.60 V 120 mV
−12 VDC ±10% (±1.2 V) –10.8 V to –13.2 V 120 mV
+3.3 VDC ±5% (±0.165 V) +3.135 V to +3.465 V 50 mV
+5 VSB ±5% (±0.25 V) +4.75 V to +5.25 V 50 mV