A123's are very hardy cells and put up with a fair amount of abuse. They have no memory and you can store them fully charged w/o issue. Indeed I charged up some stuff for an event next weekend when I got home Sunday afternoon and put everything in the trailer. All I have to do Saturday morning is pull in, unpack, and fly.
To answer your question I monitor the amount of charge being put back in after a given flights. If it changes much then I cycle the pack on a discharger to determine capacity.