Do the freezer temps go high as well? If so, the old water bottle trick might work.
Put a small amount of water in a bottle. Lay it on it's side in the freezer until the water is frozen. Then stand it up and put it in the warmest part of the freezer. If the temps go above freezing you'll have some ice in the bottom of the bottle when you check it.
2017 Eagle HT 29.5BHOK (sold)
2017 Ford Powerstroke 6.7, Crew, 4x4 (sold)
2018 Toyota Highlander
Maggie, Old English Sheepdog