You will need some way of spinning the generator at a variable rate. Short the two terminals together then attach a multimeter, set to the 50 volt scale, between the terminals and the generator case (If I recall correctly, the prewar MGs were all negative ground, so connect the positive side of the multimeter to the terminals and negative side to the case. If the generator is supposed to be a positive ground unit, just reverse the leads). While watching the multimeter, start spinning the generator. The voltage should start to build up rather quickly and you will need to stop increasing the RPMs as it reaches 20 volts (if you don't limit the RPMs, the voltage can climb very rapidly to levels that will burn the winding out in the generator). IF you see the voltage climb to 20 volts, the generator is good. If there is no voltage or it won't climb above a few volts or is it stops at 12 volts, the generator still has a problem in it. Good luck