Most of those batteries have built-in voltage regulators. The voltage stays fairly constant as the battery discharges. So you can't use it to test the battery.
Your best test might be a constant load and observing the number of LEDs that light when you press the test button at specific intervals - every fifteen minutes, etc.
I see two symptoms of impending failure with those batteries. The main one is increased self discharge. After you charge the battery, it discharges on its own. So after a couple days, or a week, it will be down an LED. Or even dead. That is a symptom that your battery is on it's way out and you may not want to trust it. Still, I've seen those batteries magically stop doing that. So it doesn't necessarily mean it's done. The second is shorter run times.
All lithium batteries should be handled gently and never dropped. That's for safety and also longevity. Storing them in a cool place, even a fridge, will extend their life. Lithium batteries wear out even if you don't use them. Ideally, store them at less than a full charge. 60 or 80%. Storing them at a full charge shortens their life a bit. When using them, keep them cool - keep them away from gear that generates heat. Heat shortens their life.