a bag of ice and an old outdoor thermometer in the water will let you check the thermostat setting.  you can put your hand on the element to see at what temperature it comes on, or preferably shut the power off/unplug and use a multimeter across the thermostat wires to see at what temp the contacts close.

I set mine about 40-45 degrees. The adjustable thermostats tend to be very sensitive to adjustment of the screw. It doesn't take much of a turn to change the temp setting a lot.

I have found cleaning the lime deposits off of the outside of the thermostat makes them more consistent and doesn't seem to need adjustment as often if cleaned once a year. I keep an old thermometer by the waterer and check the temp several times over the winter.

Jim at Dawn