The only accurate way to measure the coolant temperature (and compare it to the sending unit/gauge "system") is with a laser digital thermometer. Aim the thermometer at the head of the sending unit. Compare that reading with your gauge. The temperature of the sending unit will be higher than the thermostat since the engine heats up the incoming coolant.

Your car is 18 years old, don'y expect the "sending unit/gauge system" to read accurately at that age. Many still do read accurately, but a large percentage don't. Remember your "system" is reading the head temperature, not the thermostat temperature.

A decent tech should have one of these and they are getting pretty inexpensive on ebay. Decent ones start at around $75.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)