...damn, I'm so rubbish at this. I'm messing around with a UT30B.
I tested with a battery to make sure I understood how to read small voltages, then tested various combinations of 0V/GND and +3, +5, +12 in holes around the board, but got nothing.
So I wondered if the PSU was producing any output. I plugged the probes between the "10A MAX" and "COM" sockets on the multimeter, experimentally turned the dial to 200 in the squiggly "V~" section, and stuck the probes in a couple of the sockets on the disconnected 3-pin power cable, which resulted in a hum and a spark.
So I guess it was live then. How do I know which pin is which, and check that voltage correctly...? Any other suggestions...?
(The PSU is labelled: "Output voltage 25-0-25 AC rating 160VA, primary current 1.0A secondary current 6.4A")
*edit* d'oh, I get it, two red wires and one black one for the internal connections on the PSU. And I guess I should RTFM for the multimeter.