In the absence of suitable markings, and if they look the same the only option is to carry out a test to measure the output voltage. Unfortunately that is hard to do without an oscilloscope or a high end multimeter which can calculate pulse duty factor or true RMS.

You can do an approximate test with a 6 volt incandescent light bulb (one from a flashlight should do the trick - do not use an LED) and a 9 volt battery. The stabilizer is a 3 terminal device; input, output and ground which is the terminal connected to the case. To do the test, connect the 9 volt battery with + to input and negative to the case connection (ground). Connect the light bulb between the output terminal and the case connection. The up to 1972 stabilizer regulates at around 5.1 volts RMS so the light bulb should have close to normal brightness. The 1973+ stabilizer regulates at 10 volts so the output should be 9 volts which should cause the light to be really bright (just before it burns out).

The wiring diagram I have does not show wire colors on the instrument cluster so you will have to figure out the stabilizer input and output terminals from examination of the cluster connections.