I have a multimeter and use it fairly often, but I would not know WHERE on the PCB to test the voltages and where the voltage adjustment pot is.
I'm not sure what kind of power supply you have, but the adjustment pot (if there is one) should...well...stick out.
There are several places you want to check power: on the power supply itself, behind the harness, in front of the harness and on a chip on board. Here's why:
Checking on the power supply itself makes sure that it's actually working. In fact, as well as checking for DC voltages it's not a bad idea to check here for AC voltages as well. If there is any AC at all there's likely to be a problem with the power supply. Assuming there is to AC, note the DC voltage and move on to the next spot.
Next spot: behind the harness. Compare the voltage to the first voltage you noted. Is there a huge difference? By huge I mean more than 3/4 volt? There's a problem along the way, and you'll need to figure out what it is. There will /always/ be a drop, though how much depends on the harness and its condition. Could just mean reseating an intermediate harness - could mean a nearly broken wire. If that's okay, note that voltage and move on to the next spot...
Next spot: in front of the harness. Compare the voltage here to the voltage in back of the harness. Again, if there's a big difference, there's an issue. Likely oxidation on the pins of the harness. Take either a Pink Pearl eraser (yes, it makes a difference) or a fibreglass pencil and clean the edge connector. Also take a credit card and some very very fine grit sandpaper and buff the harness pins. Then reconnect and measure again. Obviously if the edge connector is burned it'll need to be repaired before you continue. Once you note that there is a very small difference in voltage, move on to...
Next spot: on-chip. This is one of the most critical spots for good voltage. Meter from the legs on diagonals at the ends. If you get an oddball voltage you've got the wrong pins - move to the other sides. Some boards will have a huge drop from where you measured in front of the harness, some will not.
As for where on your game you should be measuring voltages, consult the manual for where the ground and +5VDC go.