How to check the battery level?

Hey Jinsub, apologies for my very late reply. As far as I know, no. The device will just shut off eventually. I would need to create an additional circuit to tap the battery but I don't really have the wherewithal to do that until I develop a more custom microcontroller solution rather than these ultra cheap dev boards - though I'll have to be able to get a similar cost to justify it. You can do it with a multimeter since the voltage drops to I think 3.3V when it is near dead on these 3.7V batteries. Best thing to do for now is just know your relative battery life.