I use a couple of Taylor thermometers with probes (use for barbecue) and a Thermapen instant read. I bring a pan of water to a boil, which at my elevation is 212 degrees and place all probes in the boiling water. One of the Taylors is off by 4 and the other off by 6. The thermapen is dead on. I nuke a cup of water for 30-40 seconds, put all probes in the water and compare it to the temperature gauge I want to calibrate. I am using two GQF dial thermometers which only go up to 125 degrees and are adjustable. If yours are not adjustable then note the difference. If yours is a digital thermometer without some form of probe, my technique will not work. If you have a thermometer which does not go higher than 212 degrees, do not put it in boiling water.
Hopes this helps.