Lifted from Bite Size Typs:
- Get a clear plastic container (IE: soda/pop bottle)
- Add some pebbles/small rocks – 1 cm/0.5 inches worth
- Add water to cover the rocks
- Invert/Turn the container upside down and leave it in the freezer
Check the container – if rocks are visible at the bottom (top if it were right side up), the freezer lost power for long enough to be concerned about food spoilage.
Bite Size Typs suggests using a mason jar, but the glass is thicker than a soda/pop bottle where the heat transfer would be faster.