You may have seen simple voltage testers, sometimes incorporated into screwdrivers, sometimes not.
They have two connectiions.
In the case of a mains testing screwdriver, the connections are the screwdriver's tip and a small metal contact at the other end.
To use it, you put a finger or thumb on the metal contact and touch the wire, or whatever you are testing, with the screwdriver's tip. If there is voltage on it (with respect to Earth) the neon inside will strike and light up.
In the case of a separate tester, they have two wires, around 3" long, with a small metal probe at the end of each of them.
To use it, you place one probe on one wire, for instance, and the other probe on the other wire. That will tell you if there is vboltage between the two wires by the neon lighting.
Or, you can hold one of the probes and with the other probe touch a wire or connector to check if there is voltage on it (between it and Earth).
They work on the priciple that, although you may not realise it, there is an albeit high, a very high, resistance path from you to the Earth.
The small current which flows from the live mains point through the neon and its series resistance, and your body to the Earth is sufficient for the neon to strike.
If you are not convinced, buy one of those testers and make some checks yourself. You may think you are completely isolated from the Earth, but seldom is this true.