OK. Tough question since battery is VV.
Since you do not have direct access to battery itself - only thru the electronics - that is a problem.
The time it takes to charge is a good indication. If the charger is 500 mah and battery is 1000, it should take 2 hrs to fully charge, give or take a few percentage points. 1000/500=2
If the battery charges too fast or takes much longer, the battery is beginning to fail. The charger cuts off when the circuit reaches 4.2 volts. It can reach that fast if the load capability has decreased. If the charge takes too long, it is because it cannot accumulate a voltage high enough to shut it down. This empirical method only works if battery is run down to the point it shuts off, not sometime before that happens.
You can number the ego's and track individual times in a notepad occasionally. This record would also give you an idea of state of each one if you have several.
Hope that helps.