That's actually a good point. If you can, buy a charger that can test the battery. I have one of the top LaCrosse models which will (optionally) fully charge a battery, then discharge it to measure it's capacity, then charge it again.

It's how I test older cells which I suspect are near the end of their life before they fail me when I need them most. I also throw new batteries on this cycle to see how accurately they're labeled.