The most reliable way to check freezer temperatures is to use an appliance thermometer. These are small, inexpensive devices that consumers can place in a refrigerator or freezer to obtain an accurate temperature reading.
The U.S. Food and Drug Administration recommends placing the freezer thermometer near the front of the freezer. The best location is one that makes it easy to read the thermometer without picking it up, as this may affect the reading. Some thermometers may take a few hours to give an accurate reading, so it works best if they are left in place all the time and the temperature is read once a week. Separate thermometers are generally more reliable than built-in ones on most models, as the sensors for built-in models may be near cooling elements and provide an inaccurate temperature as a result.