I have several that disagree but it doesn't matter as long as I know how much they differ.

If in the thermometers range, you can check 0 C/32 F with a slurry of well stirred ice & water or boiling water 100 C/212 F.

One thing to understand is that consistency is more important than correctness. If you have been working with one (errant) thermometer and you are getting good results, and you switch to a properly calibrated thermometer that reads differently you will change your photographic results.

This is not to say correctness is unimportant.