For all you know there could be a lump of lead inside your battery (for feel-good weight) and a low capacity battery to give an output. One way to determine capacity is to fully charge it then conduct a controlled discharge, measuring current and voltage at specific periods of time. From that data you draw a curve of watts versus time then totalise your readings to get watt-hours, divide the results by average voltage to establish a rough idea of AH capacity. It would be even better if you could discharge at constant current but that is a little more complicated to do.