Using Multi-Meter

Remove the water temperature sensor from the intake manifold.
With the temperature sensing portion of the water temperature sensor immersed in hot water, check the resistance.

Fig 1: Identifying WTS Temperature/Resistance Reference Table
Courtesy of KIA MOTORS AMERICA, INC.
If the resistance deviates from the standard value greatly, replace the sensor.

Fig 2: Identifying Acceptable Deviations To Temperature/Resistance Reference Table
Courtesy of KIA MOTORS AMERICA, INC.