I used a candy thermometer (and/or meat thermometer), from the local department store. I put one inside the Incubator, and one inside the steralite container. (Because it can be hotter INSIDE the egg container than outside.
(Usually, it's only 1-2 degrees.) But make sure you try it out for a few days first, before putting eggs inside. As the Hovabator I had, had "hot spots" that were a few degrees hotter than in other spots.