Years ago we used a battery hygrometer to test the cells. The amount of floating balls determined the state of the charge left in the battery.
Yes, to do this you need a battery that has removable caps on the cells. I used to have a hydrometer
that had a float for measuring a battery's specific gravity and it had a thermometer in it for adjusting the reading according to ambient temperature. It has long since succumbed to dry rot.http://en.wikipedia.org/wiki/Hydrometer
Measuring the specific gravity is a true indication of the battery's state of charge, as well as a way to discover a weak cell.
I know there are chargers out there somewhere that will automatically read the battery's condition and charge it accordingly, you leave it connected all the time.