You can buy simple fridge/freezer thermometers... or get something higher tech like this:http://www.amazon.com/Chaney-Refrigerat ... 798&sr=8-1
I have both.
Here's my review on the Chaney (I gave it 4 out of 5 stars):
I have been using this monitor for almost three years now.
I think it's really nice, though I had problems using two units (since I have two fridges) because they seem to interfere with each other. That was disappointing.
With this unit you can monitor both freezer and fridge with a sensor in each, and you can set alarm temperatures, so you could also monitor two freezers or two fridges as they are configurable. You'll probably need lithium batteries (2 AA for each) for the sensors though, especially for the freezer since lithium batteries perform much better in cold temps. Beware though, the range is not that great, probably due to the metal of the fridge, but should work great if you stick the receiver on the fridge itself.
You can turn the alarms on and off for each sensor... and when an alarm goes off (because it is too hot or too cold based on your hi/lo settings), then it beeps every so often until you reset it by turning the alarm off then on again.
I'd give this unit 5 stars if you could buy two of them to monitor two fridges but unfortunately they interfere with each other (unless perhaps you put them completely out of range for each other).