What battery voltage?

I bought my 1971 1300cc Bug as a non runner, no battery. How can I tell if its a 6volt or 12volt? I tried starting it with a 12V battery but although it fired up it would not run on. Would only run on easy start. I have it out now and will be doing a clean up but its only done 43K so not a complete overhall. So, what voltage?

It’s a 12volt

Thanks, I take it the older bugs were 6V?

Yes, I’m sure the change over was around '67 altho you do find most early bugs have been changed to 12v at some point.