How do I check this model's power supply

iMac won't power on. How do I check this model's power supply?

History

My iMac won't power on at all and I suspect the power supply is the issue. I've searched everywhere I can think of, but no one has a tutorial on how to check it on this particular model. I was able to disassemble it without any trouble and I removed and inspected the power supply visually, but I'm hoping someone has advice on how to test it with a multimeter. I haven't been able to find any pinout or voltage information for it. Maybe the same unit is used in other models? Any help would be greatly appreciated.Three of the diagnostic LEDs are one...

I think that will not help because there will nothing turn on - no HDD or processor .....

very helpful if you could give power supply pinout, because when I tested the cable from power supply i found only 3.5V and 12V and many emty contacts