|If the system is working, the computer's bios shows the current voltage readings for the system. If there's nothing wrong with the mboard, those voltage readings are fairly accurate.The readings for what is supposed to be +3.3v, +5v, and +12v are direct indicators of the voltages the power supply is putting out and should be within 10% of the nominal values.|
Failing or dead computer power supplies are common. They're relatively cheap these days and there's no point to trying to figure out what's wrong with it if it actually doesn't work properly. Even if it is putting out all it's voltages within specs, there can be other things wrong with it.
The only sure way of determining whether your problem is caused by the power supply is to connect it to a known working system (that your power supply has enough capacity for), or try a different power supply (that has enough capacity for your system) from a known working system with your non working system