A normal voltmeter is no good. OK, then we need a way to prove that it does. Don't take my word for it, go measure it for yourself. Connect one input lead wire to ground , hold the other lead in your hand, scuff on the rug, and watch the voltage-reading on the meter as it climbs up to several kilovolts. Now disconnect the power supply, ground one brass ball, hold your finger against the other one, and start madly scuffing your shoes on the rug.
Sparks leap across the gap.
Your body charges up to 3 kilovolts, and causes a spark-gap breakdown, just like the power supply did. Mount them on insulators so the round bottoms face each other across the tiny gap. Limit the current to below a few hundred uA by using a series resistor chain. If you don't know how to use DC high-volt power supplies safely, then don't mess with them. Use the power supply to charge yourself to 3KV, 5KV, 7KV, and touch a grounded knob each time.