You can check the band’s battery from the Smart Coach screen by looking at the band icon in the top-right corner, next to the chat.
A colored line circling the band icon indicates the battery level — the more complete the circle, the higher the charge. If there is no line, the band is out of battery.
If you tap on the button, it will open the Band Settings, where you can view information such as battery level, last sync time, and other settings.