There are two ways to check the battery level.
On the unit:
Press the ON/OFF button for 1 second and observe the LED as:
3 green LED: 80%-100% battery
2 green LED: 40%-80% battery
1 green LED: <40% battery
0 green LED: 0% battery if not connected to power or 100% battery if connected to power.
The Apps for iOS devices and Android devices both have battery indicators that display of battery life remaining.