A voltage applied directly the gauge of 1.5-3 volts (1 or 2 flashlight batts) should give a mid-range reading. Full reading should be about 4.5-5 volt. That test will at least determine that they are not DOA. If you apply 12 volts to the limiter you should get an intermittant 12 volts at the output--think morse code, on off on off like a turn signal flasher. The AVERAGE output of this low tech device will be about 5 volts.