You can also get a calibration thermometer they are accurate but very fragile and they contain poisionous alcohol so you don't want to break it in your wort. And you must be very careful with it.
Then get a cheap digital thermometer and note how much the digital varies around the most frequently used temps.
Say 125, 150, 175 and 200 then you can extrapolate for the temps between your measured marks. Or if you have 1 or 2 temps you are trying to hit everytime note those exact temps and the variation between the digital and the calibration thermometer.
The calibration thermometer is great for adjusting the reading on bulkhead type thermometers.
After you make notes or adjustments put the calibration thermometer up for safe keeping.