The device sends worn status information as part of Activity Data Characteristic of Device Control Service (see datasheet section 22.214.171.124). The worn status will also be added as part of historical time series data sent via Object Transfer Service (OTS) by end November 2018.
The device can also be configured to store accelerometer data to its internal memory when ACC_PPG data is selected in the Operational Parameters Characteristic (datasheet section 126.96.36.199). The data is transferred to the app via Bluetooth using the Object Transfer Service (see section 3.6.6). Our example application does exactly this and we recommend using the source code as reference. Note that the data sent though OTS is sent as 4KB objects meaning it will arrive in batches (~30 second chunks if ACC_PPG is enabled) and not as a real-time stream.