Answer:
A 'Megger' tester is used for testing insulation. It supplies a high voltage and sees how much will leak across from one conductor to another.
Old machines used a crank handle to produce the high voltage. New machines use a battery and in inverter.
The basic test is betwen the live (hot) wire and earth.
Connect one lead to live and the other to a suitable earth point. Crank the handle or push the test button and observe the scale.
If you get a large meter movement or small number on a digital scale, you have a problem. You are looking for infinity or no movement ideally.
Do not test sensitive electronic devices with a Megger, the high voltage will destroy semiconductors.
The word Megger is derived from MegOhm, (millions of ohms), which is what you are looking for. 'Megger' is however a trade name of the original device.