If you do the ice water test, here are the directions I received from Thermapen.
The most fail-safe way to see if your ThermaPen is functioning
correctly is to test it in an ice bath. To do this, take an insulated
mug and fill it about 2/3 full of crushed ice. Then fill it with cold
(preferably refrigerated) water so that the water level does not rise
above the level of ice (you don’t want the ice to float at all). Give
it about a minute to equilibrate and then, while stirring the mixture
with the ThermaPen probe, take the reading. According to factory
specifications, it should read 32 °F, ± 2°.