Hi to all,
As had been said having two 6-V batteries connected in series is the best way to go. You'll get twice the amperage output of a 12 V battery. There is no real easy way of determining the charge capacity of your batteries unless you use a specialized current loading device which is not easy to transport and is expensive. However with a digital volt meter battery voltage can give a good indication. A fully charged battery will have 12.6 volts +/-. Make a chart and this will help. 12.6V = 100%, 12.5V = 90%, 12.42V = 80%, 12.32V = 70%, 12.20V = 60%, 12.06V = 50%, 11.9V = 40%, 11.75V = 30%. As was reported having a digital voltage gauge hooked up will update you in regards to the batteries charge condition. As you can see by the battery voltage readings a non digital meter will not accurately display these small differences in voltages.
I hope this helps.
TeJay