The Apparatus.... yeah, I like that ..

If you look at the chart I posted , you notice what the ohms of the temp sender unit should read at specified temps and the gauge should correspond .
So, if you unplug the temp sender unit and substitute a 38 ohm resistor in its place , the gauge should read 100 C.
If the gauge does not read the correct temp, you know you have an out of calibration/faulty gauge.
[ or possible poor connection]

And when the sensor is hooked back up, when the gauge indicates 100C, the sensor should read out at 38 Ohms.