Check out these Voltage Testers images:
High voltage equipment
Image by VeloBusDriver
This mechanic came out with a voltage tester to check what the problem was with my coach. Having worked in the tech industry debugging servers, this road call felt more familiar to me than when a mechanic comes out to work on a broken down diesel coach. Even then, the problem is sometimes a computer, rather than a mechanical part.
Voltage Meter Module
Image by fonitronik
selectable working range 0V to 10V, -5V to +5V.
additional cable tester.