How to Test Power Supply

Hi,
I need to test the power supply on a customers computer. The power supply is a Elan Vital model # evn-1355a. In one of my manuals it said to change the cmos battery and then try power, has anyone heard of this before. I am going to do that but have not yet. It is a workstation and is very compact and I need to figure out how to get in to the battery.
When I use the digital meter, what are the steps to test the power supply, how do I go about them and what should I see. Where do I put the wires from the meter etc… This is my first time trying to test a power supply.
Thank you.
“Have A Great Day And Look Both Ways”
Mark