The Megger test is a method of testing making use of an insulation
resistance meter that will help to verify the condition of electrical insulation. This is certainly not a new test, and has been in use for a number of years. One of the reasons that it is still such a popular option is because it is nondestructive. The test does have a limit of between 500 and 1000 volts, so it may not always be able to detect some insulation punctures. It will usually show the amount of moisture, the leakage current no the moist or dirty areas of the insulation, and winding faults and deterioration.
The best way to make sure that you have the right information and readings is to make sure that you are using the right test equipment. Fortunately, quite a bit of high quality equipment is on the market today.