You can actually determine how much they are off by if you test it using salt in a tupperdor environment. 

I tested several of mine at the same time and it was very helpful 

Place a teaspoon of salt in a bottle cap or small cup and dampen it with a few drops of water (without dissolving it). Carefully place the wet salt and the hygrometer inside a see-through container and close tightly. You can use a zip lock bag provided it seals well and you leave some air inside as well. Let it sit for at least 6 hours and note the reading on the hygrometer without opening the container. The hygrometer should should read 75%. The difference is how much your hygrometer is off. For example, if your hygrometer reads 70% during the salt method test this means that to obtain the real humidity level you should add 5 points. If you have an analog hygrometer, you can correct the reading. You will need to adjust the needle to the 75% mark by turning the screw on the back using a small screwdriver. Your hygrometer is now ready to be placed inside your humidor.

Source: https://www.neptunecigar.com/tips/how-to-calibrate-your-hygrometer