My fan always comes on at start up of electrical system on, but only great occasion thereafter.
My heater pads click on below I think 35°+-.
Turn yours on.
Feel them for heat.
This is a sure fire way to tell
In case yours aren't thermostatically controlled, the link is an easy solution that provides valuable information.
(one of these is going in my fridge vent in case it gets too hot in there. Probably unneeded, but 8 bucks, 12v already available in the vent, and the handful of old computer fans I have is a reason to do it.)