It's possible to bench test but not absolutely simple. You need a fully charged battery and rheostat - or a variable DC power supply. You need to connect a field coil or find a suitable load for testing (a tail light bulb would work I think, and use a lot less power than a real field coil).
You want to vary the voltage on the black power input wire (with the rheostat or variable DC supply) and measure input and field coil voltage. The field coil should be powered fully (within 0.5V of the input voltage) with the input voltage below about 12V, with the coil voltage reducing as input voltage rises and fully off when input voltage reaches somewhere around 13.5V.
Different regulators work different ways - it may smoothly reduce the coil voltage, use PWM with the on pulse width narrowing to reduce coil power, or go down in steps.
I would avoid going much over 18V input, the voltage rating of the regulator's internal components is unknown.
If you never get coil power I would say the regulator is dead. Same if it's always ON but with undercharging that's unlikely.
The R/R's two black wires should be internally connected to supply power to the field coil all the time. This type of regulator controls the field coil "ground" wire, so you need to measure coil voltage across the field coil wires - measuring the coil black wire to ground will always show input voltage.
You apply input power between the black (+) and green (-) wires. The red wire is the rectifier output, it isn't used in the regulator.
You should also test the rectifier diodes, there are many posts describing this process with a multimeter.
