You can test all but the ammeter with the following procedure:
Using a 5 volt power supply, you can test them by applying positive 5 volts to each gauge. They should deflect to their maximum indication. The negative goes to the metal frame.
The ammeter can be checked by flowing current through it from the power supply to a load. Since they measure around 20 amps or so, you will need a high current supply and a high wattage load to see much deflection. Most issues with ammeters are poor connections and carbon tracking from arc overs and can be checked with an ohm meter instead. It should read infinite resistance to the frame ground using the 1,000 ohm scale.
Craig