How can know for sure

My system is a 6 volt system and spent the last several years as a negative ground system until I changed it back to a positive ground system 4-5 weeks ago. So yes it can be negative ground and 6 volt.

Stan

IF you have an alternator, then you're pretty much stuck with a negative ground unless you spend big bucks and get one that works with a positive ground.

The ballast resistor makes it look like you have what I call a "half breed" 12V primary circuit. It uses a 6 V coil with a current limiting resistor to keep from frying the coil and the points with a 12 V electrical supply. The resistor should not be used if it's a 6V system since it reduces the spark energy. Pull the wire out of the top of the distributor, hold it near a good ground and try a start. What's the spark look like? Since you have a separate voltage regulator, you must have a generator. Check the voltage regulator to see if has any markings that tell you what voltage your system is set up for. Or call the previous owner and ask him. I would think that you ammeter would show a high current if you were running with a a 6 V battery in a 12 V system. The 12V generator would be trying to charge it. You can take off the ballast resistor and see if it idles and runs better. You won't hurt the coil if you do that for just a few minutes. If it's really a 12V system then removing the resistor should improve the running across the board.

Those are my thoughts on your dilema Hope this helps.