Using a mech, if you start with a fully charged battery you'll notice the voltage drop as it depletes. It just gets less fiery & slows down. You need something to check up on your batteries, i use a multi meter & when they hit around 3.6v, that's it for me, bottom line & i recharge. I can detect it before then though.
You can take them lower but shouldn't go too low so as avoid upsetting the chemical balance of the battery. Once they discharge too much, you can't easily boot them & in theory they're finished. You'll soon recognize it if you use a mech & an rda.