I have no idea how you'd check calibration of the dash mounted thermometer. However, the other one, inserted into the vent, is very easy to check and calibrate.
Take a small glass or cup, deep enough to submerge the thermometer probe, pack it with crushed ice to the top and add cold water. Submerge your dial thermo probe almost up to the gauge and let it sit for a minute or two. The slurry will be 32 degrees F. so, after a short while, that's what the dial should read. If it's not reading 32, look at the back of the dial and there should be a nut, with flats, between the dial and the probe. While holding the nut stationary, and the probe still submerged, simply turn the dial, in whatever direction is needed, to make it read 32 degrees F. on the nose. All done! Your thermometer is now calibrated and reading accurately as verified by the 32 degree water/slush. If nothing else, I always like to see how close mine are to being accurate at 32 degrees. They have a tendency to creep over time or with accidental shock and rough handling.