Comparing wet bulb and dry bulb temperature is the most accurate, low cost method for measuring Relative Humidity.

Simple, direct reading hygrometers are available but vary widely in accuracy and are not recommended for incubation. The Brinsea wet bulb thermometer hygrometer is specifically designed for use with the Octagon 20 and 40 incubator or Octagon 20 parrot brooder module and comes with conversion chart showing Relative Humidity.