You can see the battery level when you open the Neosensory app or on the Device Management screen of the app. If you prefer not to use the app, plug in your Buzz to charge. The battery level will flash briefly when you plug it in and again when you unplug your Buzz. One light means low battery, two lights mean medium battery, and three lights mean fully charged.
If your Buzz is very low on battery, it will flash one red light three times and vibrate in a sweep around your wrist to warn you.