There are two ways to determine whether a high voltage is present in a circuit. One is to spit on your thumb and index finger, place them across the circuit and wait for the jolt.
The other is to hook up a voltmeter.
The voltmeter is a classic instrument, still indispensable despite its age (the magnet-and-coil mechanism on which it is based dates back to the galvanometer, introduced by the French physician and physicist Jacques-Ars