You can check the battery level in two different ways when the headset is powering on.
When the headset is powering on, the red LED flashes rapidly indicating the battery level.
Voice Prompt Indicator
When you power on the headset, keep pressing and holding the (+) Button and (–) Button simultaneously for about 3 seconds until you hear three high tone beeps. Then you will hear a voice prompt indicating the battery level. However, if you release the buttons as soon as the headset turns on, you will not hear the voice prompt for the battery level.