The easiest way to determine the accuracy of your hygrometer is to perform a salt test. Mix water and salt together to make a paste (no excess water free floating on top) and place it in a large sports drink cap. Place the cap and the hygrometer inside of a Tupperware container, seal it, and leave it for 24 hours.
Once the time is up, if you receive a reading between 73-76%, your device is operating accurately. A perfectly accurate reading would be 75.0-75.5%, however it’s important to remember that hygrometers have a +/- 5% deviation.