A little more information please. Long time since I worked on a Beetle with the engine in the back so details please. Pictures would be good?
Back to the lights, some of those really old ones had 6Volt electrics, but thats going back to the beginning. You probably have 12Volt? If in doubt the dynamo and starter motor will be stamped.
Make yourself a test light with two leads or as already said use a multimeter. Switch the headlight on and you should have battery voltage across two wires behind the bulb. If no joy connect one lead to the chassis and check for voltage on the wires behind the bulb.
Let us know how you get on.