I installed a small coulometer (with LCD-display, available from Ebay) between my charger and battery. I can also discharge battery (with car tail light bulb as load) with coulometer. For example, before disconnecting charger I can see that charger put 3 Ah to battery. Annual test shows total capacity from 100% charge to 0%. No more guesswork about battery condition.

There would be market for full-blown panel mounted battery monitor system for gliders. One that monitors each battery charge and discharge, with built in breakers etc.