Boiling water test. Get a pot of water (with no salt) up to a roaring boil. Then stick the thermo in there and wait for it to settle in to whatever temp it reaches. Once there, adjust it to read 212*F. That should get you to within a +/-5* range while cooking.
18.5" OTS, 22.5" OTG, CB Silver(Rip), Vintage Coleman Bullet, Pit Barrel, UDS