Set your meter for measuring DC voltage. See attached image (or previous link) for locations of the test points.
Black lead on GND, red lead on the +5, +12, or -12 test points.
Make sure both fuses are good, make sure the fuse holders aren't fatigued and are making a firm connection.
There is no stand-alone power supply used - the circuitry providing the above voltages is built into the sound board. Once you know what voltages you do/don't have, you'll know whether it's involved in your issues, or if there is something else (IE a bad audio amplifier).
The pinwiki link goes over this; if you're not comfortable proceeding once you check the voltages, at least you can send the board off for repair with some knowledge of the likely issues.