Just about any motherboard manual will have the pin asignments for the power connector.

I have a little load block that when plugged into the connector turns the power supply on and puts a load on it (switchers may not run at all with no load).

A realistic test is to put the power supply on the bench plugged into an old motherboard and hard disk. A power switch from an old cabinet can be used to turn it on and off (or just a screwdriver momentarily shorting the power switch pins on the motherboard will do).

Use a good multimeter with thin probes and you can just stick the probes down into the connector from the top alongside the wires.

Of course voltages don't tell you other important stuff like ripple (some meters will give you a decent ripple reading when set to AC) and noise.

The ATX spec can be found on the Internet.