How do I know if a sensor is online?

If a sensor is online, its LED is off and it will be displayed as online in the app (go to the SYSTEM OVERVIEW in the app, select the sensor and click on HELP to view the status). If the LED is flashing, the sensor is outside the range of base.
If the sensor's LED is off, but the sensor is shown in the LED as offline, it is very likely that the batteries are flat.