Flip your heater on- see if the temps stay the same, if there is any coolant loss.
FWIW, for most of us at least, our cars actually get hotter when you turn on the heater. It can be as much as a 10+ degree swing when on vs off. It's odd, but just how the coolant temp sensor is located in relation to the heater core and the thermostat. There's a few theories on why this happens, but it's still odd for those unfamiliar who are used to the opposite affect from previous vehicles. So if you run the heat and see an increase in temp don't be alarmed it's actually fairly normal.Great idea!