If you have a standard pocket cooking thermometer you may have noticed it has a hexagonal nut on the reverse side of the dial, which allows for easy calibration. Use a cup of ice water to make sure it gives the correct temperature.
Chef Keith Snow of the local food culinary weblog Harvest Eating suggests that if you don't have another properly calibrated thermometer on hand to measure your cooking thermometer against you can always fill a cup completely with ice, add water, and stir the ice and water together for a minute to equalise the temperature.
Many pocket thermometers come with protective cases and many of those cases have a small built-in calibration wrench for the thermometer. Place the thermometer in the cup of ice water and adjust the hexagonal nut until the thermometer reads zero degrees C.
Using the ice water method may not give you a perfect laboratory-suitable calibration, but it should be accurate within a degree or two and very suitable for cooking use. If you've had the same analogue cooking thermometer for the past decade, you may want to take it out and give this trick a try.
How to Calibrate a Thermometer [Harvest Eating]