Turn the refrigerator thermometer to the "On" position. Set the refrigerator thermometer to read Fahrenheit or Celsius by manipulating the selector switch located on the refrigerator thermometer.
Place two refrigerator thermometers in the refrigerator next to one another. Allow 30 to 60 minutes to pass, and then take the reading of both meters by noting the temperature display.
Compare the temperatures displayed with one another, along with the temperature reading on the built-in analog or digital thermometer to establish a baseline.
Read the refrigerator thermometers in increments of one-hour intervals for a period of four to five hours to see whether the internal temperature is consistent. Adjust the refrigerator's temperature regulator so that the temperature is under 40 degrees Fahrenheit.