If you have a Heater / AC control that is the size of the radio with a display in the middle, the procedure below will work. If you have 3 knobs it won't.

While the car is running, press and hold the REST button until the display starts to flash 01 and some other number. Press the + button on the fan speed until you see 05 or 06 ( can;t remember ) This is the coolant temp in degrees F. 197 is what my 97 C280 and 97 SL320 will see on a hot day with AC on. To clear the data function either turn the car off or press and hold rest again.

80 C is 176 F and kind of cold.
85 C is 185 F is about right on an average day
91 C is 197 F

A coolant change is a good preventative measure as would be a thermostat. With the age and miles, it is reaching the end of reliability.

You really need to find a trusted non chain store shop and have them care for the car. Chain stores are usually only interested in taking high profit margin quick turn over easy work. ( Brakes, tires , wiper blades, battery, hoses )