Fill a milk bottle cap or other small container with salt, and add a few drops of water (not enough to dissolve the salt).
Put the cap inside of a baggie or plastic container along with your hygrometer, and seal the bag.
Wait 6 hours, then check the reading on your hygrometer without opening the bag (or immediately after removing). If the reading is 75%, then your hygrometer is accurate and no adjustment is required.

If the reading is not precisely 75%, then adjust the hygrometer to 75% by turning the screw or dial on the back. This must be done immediately after removing from the bag or container, before room conditions cause the reading to change.
If there is no screw (or dial) to re-calibrate your hygrometer, then you will just have to remember to add or subtract the difference between the test reading and 75%, in order to determine the actual humidity level inside of your humidor. For example, if your hygrometer test reading was 80%, then subtract 5% from the readings you get inside of your humidor, to determine the actual levels of humidity (e.g. a reading of 70% inside your humidor equals an actual humidity level of 65%).

Hygrometers should be tested at least once a year, and re-calibrated if necessary. When I tested two of my analog hygrometers at the same time (see photo), the cheap plastic one measured 12% under while the better chrome model measured 2% over. Although I re calibrated both models, I plan to replace the cheap one with a new digital hygrometer. If you invested in a good humidor, do not risk storing and aging your cigars improperly by relying on a cheap or faulty hygrometer.