How to check thermometer accuracy??

If it has a stem, Fill a zip bag with hot tap water. Measure the temp with a known accurate therm. Wrap the water bag around the smoker therm stem and hold there until the needle stops advancing. If the temps match, it is accurate. What water temp you use does not really matter as mutiple decimal place accuracy in smoking is not critical...JJ

If it nails 212F for boiling water, that single verification should give you some confidence that it is at least accurate in that general temperature range.

I run a testing laboratory and we have many calibrated thermometers.  When they are out of calibration at one or both ends of the range, or by more than 1% at any part of the range, we pitch them.  BUT, when we graphically plot the calibration results they are usually linear between the checked temperatures even if they are not reading accurate and precise.  

For instance...Even if the thermometer is spot on at boiling (212F) off by 5 degrees at freezing (32F) it would only be off by one degree at 176F and at 248F and even less so between that range.

Ed