So, I found the IBM service manual
http://ps-2.kev009.com/ohlandl/books/ps2-hmm.pdf
Says the PSU is 27f4166.
There's also a power connector pinout. So far as I can see there is no means for the motherboard to signal the PSU that something is wrong, so the PSU should run with nothing connected to its connector and the voltages on it can be checked. Pin 1 is the "Power Good" line which the PSU signals *to* the motherboard that it is okay for it to start.
So try just turning on the PSU with nothing connected to it, and in theory if it keeps running it is okay, and if it doesn't it is defective. Pin 1 should switch to 5V within a very short time of switching on.
Also, do just check that it is set to 230V.