A candy or bulb thermometer can be used to check the temperature inside of a refrigerator. There are also thermometers specifically made to be used inside of a fridge and work properly in the necessary temperature range.
A thermometer not made for refrigerator use should be put in a container of food or liquid in the fridge that is already cooled. The container should then be placed in the middle of the fridge for an accurate reading after about three minutes.
Refrigerator-specific thermometers can be placed in other locations and still get accurate measurements. Some of these have labeled dials that list safe temperature ranges.