Get a pot of water boiling and stick the thermo probe in. The thermometer should read close whatever the boiling point of water is at your altitude.
Some dial thermos have a screw that can be used to adjust the pointer to correct the error if there is any. Most digital thermos can't be adjusted.
Here is a web site that will calculate the boiling point at different altitudes.http://www.csgnetwork.com/h2oboilcalc.html
Some people also make an ice slurry (crushed ice in water) and check to see how close it reads to 32°. Most inexpensive thermos have a "sweet spot range" that they are designed for so they are more accurate in the range that they are meant for like cooking. And their accuracy or inaccuracy is not linear. For this reason, I don't bother with calibrating at 32°.
Hope this helps.