If I had the project to test your Voltage Regulator, I would hunt down a 0-30 volt variable power supply with at least a 200 milliampere capacity. Connect the positive lead of the power supply to the Bat contact on the regulator and the negative lead to the regulator case. Connect a 100 ohm 5 watt resistor from the power supply lead to the field contact of the regulator. As the variable power supply is brought up measure the voltage with a DVM from the field contact to the VR case. It should read ~0.7 volts. As the power supply voltage reaches ~13.5 volts, the field contact voltage should pop up to the supply voltage. That should check the operational status of the regulator.
Another test to perform is of the shunt diode that is in the regulator. If your DVM has a diode testing function, check the forward and backward resistance of the field terminal to Batt terminal. The resistance with the positive lead of the DVM on the field contact should be significantly less than with the negative lead on the contact.
That is about as much as you can do to check VR. If you decide to destructively examine the VR, remove the cover and see if there is an adjustable potentiometer on the circuit board. I have always wondered if there was one inside.
Hope this satisfies your curiosity.