wondering what is the best way is to check six volt batteries to see if they are getting old and week

How about discharging them on a device that measures capacity and voltage? The CBA battery tester works good for this along with some higher end chargers.

I assume you are speaking of 6 volt flight packs. Many chargers on the market tell you how much current is replaced in the battery when it is charged. A good way of knowing what the pack is doing is to discharge it, using a load that creates a discharge rate that is safe for the pack. A couple of cycles of discharge and charge for nicads gives you a good idea of how much capacity the pack has.

This day and age, with the new battery technology available, the old wall wart kind of charger is just not sufficient. The price of a charger that really does what you need, including providing charge and discharge information, is much cheaper than the cost of a ruined model.