Those amp gauges aren't really anything more than ornaments. No real meaningful measurements can be gained from them other than a load indication. If yours bounces slightly and deflects slightly in the negative direction when you turn on headlights or heater fan then it is probably working. The voltage gauges that replaced the amp gauge later on were a better indication of an operational charging system.
But if you'd like you can test the amp gauge, out of the car, with a 50mA load and get almost full needle deflection - reverse polarity and check deflection in the other direction. Use the WHITE and WHITE/RED STRIPED wires.
RLS30