Some of the advice is way too complicated for this test.

Simple, remove the ECT from the head, and with the sensor comnnected to the cable in the harness, prepare a beaker or cup of ice water, and put the sensor in. Measure the resistance at the connector for the ECU. Repeat with a cup of boiling water, measure resistance again.
As a third point of comparison on the curve, measure resistance at the ambient temp.
Compare the three points to the curve of resistance in the Greenbook or Bently's.
Any small probe measuremt difference such as the ambient or boiling water will show small offsets to the curve, atmospheric conditions, elevation etc, will not change the shape of the curve, so don't worry about it. Any mercury thermometer or TC probe on a DVM will be more than adequate for reference, but the true reference is the hot/cold junction point, not the probe or measurement instrument.

If the resistance measured at the ECU connector shows a uniform offset in the direction of increased resistance at all three points then the wiring or connector may have corrosion, causing a decrese in continuity.

If the curve made by your three measurement points show the low temp and mid point temp offset in the higher resistance direction, then the sensor is fading.

Double check the temps with the sensor immersed into each bath without being connected to the harness. Subtract the two sets of data and the result is the wiring and connector component, which is usually 90% of the problems with these sensors, IMHO.