Air humidity can be estimated by measuring

• the dry bulb temperature
• the wet bulb temperature

Dry Bulb Temperature - T_db - can be measured with a simple thermometer as shown above.
Wet Bulb Temperature - T_wb - can be measured with a standard thermometer with some wet clothing, cotton or similar, around the bulb. Note that a continuously air flow is important to evaporate water from the wet clothing and achieve a correct wet bulb temperature.
Sufficient air movement can be achieved with a sling thermometer or similar.
Relative humidity can be estimated from the tables below or alternatively from a psyhrometric or Mollier diagram.

Temperature in Fahrenheit

Temperature in Celsius