I meter at roms, but on the Vegas there's hardly any difference in voltage at all between the roms and the hard drive plug on the board. it's easiest if you plug the hard drive into the game board and you can't use a separate hard drive plug coming off your power supply for this test.
with the hard drive plugged into the board, there's 4 wires on a hard drive power connector: red (+5V), black, black (both grounds), and yellow (+12V). set your meter to 20V DC range, put the red probe into the red wire hole, and the black probe into either of the black ground wire holes. it's probably easiest to do this with the game off if you're not familiar with how to jam the probes in there, just so you don't accidentally slip off and hit something else.
a fun little game I play is I check what my voltage is coming off the power supply first, then I do the rom/hard drive plug check. if you have a substantial drop in voltage between the power supply and your game board, then you're probably looking at a cruddy edge connector, which as I mentioned before you can clean pretty sufficiently with just a pencil eraser. do each contact up and down first, then go left to right across the entire edge connector. wipe the little eraser fragments off the board, you don't want those getting stuck anywhere.
then check your voltages again and hopefully be amazed at the improvement.
you can check your power supply voltage with the red and black wires too at the 9-pin molex plug (for box type power supplies) or at the +5 and GND or COM lugs on a screw terminal power supply.
if you have any questions PM me.