How can you tell if a freezer rose above freezing?
The University of Ohio extension publishes a clever tip:


  Place two or three ice cubes in a plastic freezer bag and seal. Keep this in the freezer at all times. In an upright freezer, you can have a test bag on each shelf. If there is a power outage you will know if the interior temperature was above 32°F if the cubes melt. If the cubes are melted, quickly determine the temperature of the water in the bag and you will know the temperature inside the freezer.


Of course, they also recommend having a proper freezer thermometer.