A thermometer should be checked when brand new, and rechecked every six months or so, after you drop or otherwise traumatize it, when you haven’t used it in a while, and whenever you just aren’t sure it’s telling you the truth.
Boiling Water Method
Materials needed: a deep pot and water
In a deep pot, bring water to a rolling boil. Dip the thermometer stem two inches into the boiling water without touching the bottom or sides of the pot. Wait for 30 seconds and the thermometer should read 212°F.