Compare the wire colors with a known standard ATX power supply and see if it all matches.

To be more precise about it you could also use a multimeter to measure each pin, but comparing the wire colors should be enough.

shamino wrote on 2023-08-14, 04:55:

Compare the wire colors with a known standard ATX power supply and see if it all matches.

To be more precise about it you could also use a multimeter to measure each pin, but comparing the wire colors should be enough.

I don’t have a standard atx PSU to compare to, but if I knew which point to measure, I could compare it to a standard 20 pin drawing. Do I just measure dc volts between each pin and a ground pin?

That would work, but it's easier to just use the case of the PSU as the ground.

If the matching motherboard is still handy, you can use that to power it up, then poke the positive probe into the back of the connector to get into contact with each pin.
If you can't hook it up to the old board - there's a way to force the PSU to run all by itself, but I don't remember which wires have to be shorted. Testing that way means there's no load on the power supply so the voltages could get a little funny.

Thanks for the reply! Here is the ugly story. I got ahold of a diagram with a 24 pin and 20 pin side by side. All of the voltages match for the 20. I could not get the Dell to power on using pin 4 (power on) to a common. I was able to do so on a modern ATX PSU using a fan as a load and it worked. The last 4 pins had a break away so it fit into the MB perfectly. I also plugged in a floppy and the HD from the dell and powered them using the same 4 pin Molex I used for the fan load. Powered it on and voila, the bios came up, but did not see the HD. A few minutes later while I was flubbing around in the bios, I heard a pop, sparks started to come out of the floppy drive and it began smoking. I quickly shut it all down. Then, I unplugged everything except the main 20 pin PSU and as expected, the bios popped right up. In the meantime, I had fried the Dell HD and the floppy. Later I measured the voltage on the molex connectors and as you would expect, there was 5 and 12v. I must have somehow put the connector in the wrong way. Oh well, at least I know the modern PSU will work. Still, I’m going to buy a NOS, era correct PSU. Live and learn.