I plugged an oil radiator heater into a thermal plug out in the greenhouse. It has two sockets, so on the other one I plugged in the string of LED xmas lights that I put up last year. This way if it gets cold enough that the thermal plug turns on the heater I can tell because the lights will be on.

It isn't a gro-light glow, so I don't think the sheriff will stop by to see what I'm growing out there. (Basil, broccoli and onions.)