A quick test would be a deflection test in the car. With the key turn to the on position, pull the headlamp switch, turn on the blower motor, and if you have power windows push the power window switch button. With all this draw the ammeter should deflect toward the negative side. If the gauge doesn’t deflect then you have either a gauge issue or a wiring issue. If it doesn’t deflect, clean the wire terminals at the horn relay and make sure they are clean and tight, test again.
If the above fails, next I would remove the gauge cluster housing and test the actual gauge, this is done by testing continuity between the upper and lower studs. If you have it, then proceed below, if you don’t have continuity then the winding inside the gauge is bad.
The most accurate way to bench test a gauge is with a power supply. Turn the amperage all the way down and set the volts to 1. Hook up the positive wire to the top of the gauge and the ground to the lower. Then raise the amperage and the needle should follow. To test the gauge in reverse just flip the connection and test again the gauge should read opposite.
Another way is to lower both the voltage and amperage on the power supply and then connect the power to the top post and briefly touch the ground to the lower. The gauge should deflect. Reverse the terminals to test in the opposite direction.
And lastly, the shade tree way to test… use a small AA battery and solder wires on the top and bottom. Then connect one wire to the top stud and then briefly touch the other terminal to the lower stud, when done the battery should deflect. Reverse the connections and it should deflect in the opposite direction.
A fluttering gauge is usually a loose connection at the horn relay on 63-73.