I'm not sure if there's a standardized way of doing this or if it's possible at all. Have you Googled a bit and if so what did you find? I'm gonna offer a few links myself as they might lead to something:
First of all I'd look into the android.bluetooth.BluetoothHeadset class. It will list connected headsets as generic BluetoothDevices. From there it seems like you can issue vendor specific commands to get different kinds of sensor information (like battery level).
Check this blog post from a Plantronics employee regarding Plantronics headset sensor access from Android. There's even an example app with source code.
Check the Sony Add-On SDK for interfacing with certain kind of headsets using their SmartApp system
