There are two instruments which measure dew point.
The first is a hygrometer. This tool is made of human hair. Hair lengthens and shrinks with humidity, growing longer in moist air. A pointer attached to the hair changes position along a scale as the hair gets longer and shorter.
The second instrument is a psychrometer. Two thermometers are used, one kept out in the air and one whose bulb is immersed in water.
The dry thermometer measures the air temperature.
The wet thermometer measures the air temperature cooled by the rate of evaporation, or the ability of the air to take up more water.
The relationship between the air temperature and the difference between the temperatures of the two thermometers is plotted on a relative humidity chart, giving the percentage of moisture in the air.