You can't do that with a standard volt meter. If you have a timing light you can clamp it on each wire and just watch the pulses. Alternatively you can pull the wires off the plugs one at a time to see if it changes but you might get whacked. It won't kill you, but you won't like it If you have some heavy rubber gloves, you should be ok though. Alternatively, you can pull a wire, start the boat and see how it idles, then put it back on and repeat. This you could do from the distributor cap end. In either case you could do it at the cap end, but I don't like pulling off the cap while it's running because you've got all those moving belts so close to the cap and if you do jump, you could end up with your hand in the alternator belt.