How do I know if my heater is working?

One of the tanks I bought came with that. I do not like it very well as I cannot adjust the temp myself. I did notice once when I took it out of the water briefly that a red light came on. Other than that no lights at all. The best way to tell is to go buy a thermostat. They are $2-$3 you will want one anyway so that if you think the water temp is fluctuating you can double check with the thermometer. Good luck and have fun.