Touch the wire from the gauge to the sending unit to a good ground. The engine block or cylinder head makes a good test point, as long as you know the engine is grounded back to the battery properly. The gauge should read full "hot". Now disconnect it, and the gauge should return to the "cold" indication. If the sending unit has the wrong resistance for a particular temperature, the gauge won't indicate accurately. If you can't get full travel on the gauge needle by grounding it, your dash unit has problems.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!