Look in the manual for a temperature to sensor resistance chart.

1) To test the sensor for accuracy:

Get a single pole / double throw ( SPDT ) switch ( A 3 way household switch would work ), some wire , terminals and an Ohm meter.

Assuming a single wire sensor, connect the sensor to the common switch terminal ( black on the household switch ) . Connect the + lead of the ohm meter to one pole of the switch then the previously removed car side sensor wire to the other. Attach the Ohm meter - lead to ground.

You can now switch between using the cars temp gauge or the Ohm meter as a gauge ( with the assistance of the temp to resistance chart ) .

2) Conversely, if you got a resistance substitution " decade " box you can test the gauge for accuracy.

In this case, wire the car side of the sensor harness to common , the sender to one pole then one wire of the decade box to the other. Be sure to ground the other wire from the decade box.

Even if you got 4 single resistors and manually put them in circuit to test range, that would be helpful.

I'd try to drive the gauge to the first graduation, something at operating temp , something half scale and something at last graduation.