I happened to have a model railroad transformer, that had a rheostat that could go from 0 to 24 volts. So I set it to 12V for testing. I quickly turned it down and back up a little to see if the gauges responded. I didn't go to the trouble of trying to calibrate them, just wanted to see if they seemed to be functional before installation.
Oh, I should mention, I did this when the gauges were all still attached to the circuit board. I hooked up the power and ground to the correct pins, so the fuel gauge acted as the VR sending 5V to the other gauges. But it would work just as well on bare gauges. But this way I could also test the lamps at the same time.