Using Multi-Meter

WARNING: This page is about a different car, the 2004 Hyundai XG350. However, it is still accessible from the selected car via links, so may be relevant.

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

Fig 1: ECT Sensor Temperature/Resistance Table
Courtesy of HYUNDAI MOTOR CO.

Fig 2: Checking ECT Sensor Using Multi-Meter
Courtesy of HYUNDAI MOTOR CO.
If the resistance deviates from the standard value greatly, replace the sensor.

Fig 3: ECT Sensor Temperature Vs. Resistance Graph
Courtesy of HYUNDAI MOTOR CO.