Hi Otto,
The chargers that I use all have a numerical display that shows various details. The most important indicator to me is the mA extracted during a 'total' discharge after being fully charged (it's one of the preprogrammed cycles I can choose). That shows the total discharge capacity that can be used, and I match the best set (4 for my flash) from the bunch, then the second best set, etc.
I keep them in battery boxes where I can keep them together in the order of discharge capacity (+ up means charged and ready for use, + down means used and charging required). Sometimes the batteries come with sleeves for 4 batteries. After some time of use I just run the batteries through a charge-discharge-charge cycle, and sort them again in order of highest to lowest discharge capacity. That should also eliminate any memory effect, however slight it might be, and cells that deteriorate faster than normal will automatically end up at the rear of the bunch together with similar performers.
Cheers,
Bart