A standard voltmeter is the tool.
I've measured at every terminal on the back of the switch using the chassis/bolt as a ground. I realize that the Bat terminal should always have 12v if I remember right since there is no switch between the battery, ammeter, and ignition switch. To be thorough I checked the voltage at the fuse panel and it was still receiving 3-4v from the switch when closed. When open everything read 12+ volts.
