If you wear a Libre 2 you can check by scanning with the FreeStyle LibreLink App. Open the FreeStyle LibreLink App and click on "Scan Sensor" and simply hover your phone over the sensor. If for some reason the sensor isn’t working, the app will display an error message or give you a glucose reading value of 0. Shortly after scanning your sensor with the StreeStyle LibreLink App, you will see the latest data in your Hello Inside App as well.

If you are located in Germany, Austria or Switzerland you are wearing a Libre 3. In that case, the data gets transferred to your apps automatically without hovering the phone over the sensor.