The first time the unit is set-up you'll see a breathing white light. After 1 minute you'll get your first reading (you may need to refresh your app/page to see the readings). After that, every 15 minutes the sensor will automatically send readings (and alerts if they are set-up) which can be viewed on your phone/computer. If you want to get an instant reading simply press the right button one time (with USB facing up). The LEDs will flash red, green/yellow, and ultimately will start breathing white once a reading is sent. You can do this a maximum of 3 times every 15 minutes.
In addition, every time your sensor sends up a reading (again, once every 15 minutes) you'll see a red LED light turn on briefly. This is normal.
Finally, you can see if the sensor is working correctly when viewing your sensors in the dashboard. On the Android app or webapp you can view the "Online" column with a blue dot indicating online and a black dot indicating offline (see below).
On the iOS app you can see a blue or red dot next to the sensor's name on the Sensors tab.