For this reason, you should always put a coin in your freezer

A Coin Hack for Refrigerators—Does It Work?

Although it is quite effective with the freezers, the refrigerator works on the same principle; you could put a water dish containing a coin at the refrigerator shelf. Water will melt and the coin will fall if the refrigerator warms up during an outage of power, but you can see it has changed due to the fallen coin.

However, this adaptation is not very reliable because refrigerators do not keep things frozen. It can still be a useful tool in emergencies, but it is best used in conjunction with a fridge thermometer for better accuracy.