The leakage current of varistor has a special instrument to test, such as Taipei Rhythm Tech's TTK168 tester, the general test principle is to use 80% of the specification voltage value through the varistor after the current measured, generally not more than 20 microamps. In addition to leakage current should also test the voltage of the pressure-sensitive voltage, as well as the nonlinear index, and these are just routine tests, but also need to do some safety regulations trustworthiness test.
Thermistors do not need to test the leakage current, you need a constant temperature, such as water tank, but not water, to use silicone oil, generally 25 degrees, but also a precision multimeter, used to test the thermistor resistance at 25 degrees, but also need to do the thermal shock test of the thermistor.