How to determine if power supply or power switch has failed?

I have a late model dual core Power Mac G5 model number A1117 that will not boot up or power up.

It has been sitting idle last operated New Year's of 2011 without any issue.

When I tried to power it up last week from my wife who would like to use it now, initially the white LED above the power switch came on but nothing happened. I tried to hit the power switch again and the second time nothing happened not even the LED above the power switch. I tried unplugging replugging etc. but still nothing.

After reading through the forums I have since changed the backup battery, reset the SMU switch [my version does not have a PMU switch] and all of the steps associated with the answers given for other questions. Still nothing is working and I need to determine if the power Switch is faulty or if there is an issue with the actual power supply.

Can anybody help me isolate the problem?

Thanks,

-David