Let it run for 20-30mins then put your hand over the exhaust vent it should be warm otherwise remove bottom vent and feel the metal tube on the right, like a chimney it should be warm, have your engine running when you do, if any shut off relay for the 12v is fitted it won't activate and if none fitted your battery won't go flat....... It would take a day to get this inside of the fridge cold.
With a multi meter you can just see if the heating element has power to it but the let it burn method shows it is working.