How to test an *ancient* power supply?

In How to test a power supply, the answers focus on testing ATX power supplies.

I have a pre-ATX power supply. How do I ...

• Safely test that the extracted old PSU is not producing expected voltage?
• Safely test the replacement PSU is producing good voltage before installing it?

By safely I mean with minimal risk to PSU and with no risk to server's motherboard etc from this preliminary test. I'm looking for a minimal test for reassurance / quick check, not full-load testing.

Generic answers are probably more useful for other people but, in case it helps, the details of this are:

It is for an Intel Pentium II server.
It is a 325W PSU.
The connectors are mostly conventional but it is proprietary (Compaq Proliant 1600)

The long lead goes to a front-panel "power on" LED and to a latching on/off switch. I can extract the switch and connect it to the end of the long lead for testing).

I have multimeters and can rustle up small test loads (fans, 10 W 10-20 Ω power resistors, etc)

Solution 1:

I believe this is an AT (not ATX) PSU with switching at the PSU. These (photo) are the leads for the power supply switch. You can connect them to a properly rated switch or jumper them but, if I remember correctly, this is a Double Pole Latching Switch for mains level voltage, so (this is not a joke) try to not kill yourself.

This differs from the newer ATX scheme which uses a low voltage momentary signaling which requests that the motherboard shutdown (press to power on/off; hold for 4 secs to force shutdown)