Not as hard as you think. Before you yank them, get a multimeter. Put the negative lead on the negative terminal of the battery, set the multimeter to resistance, and test both ends of each ground. They should read pretty close to zero. The one that reads above 10 ohms or so (just a ballpark) is the one with a bad connection, or if it reads "infinite", its disconnected or broken.
If that sucker's reading 1/2 less than it used to, I'm now 99% percent sure it's a ground or voltage issue, probably the one right next to it on the t-stat housing. The temp sensor for the gauge is nothing more complex than a "thermistor". It is a variable resistor that changes it's resistance with the temperature. The colder it is, the higher the resistance. Lower voltage or a bad ground on the motor (which can be translated as "higher resistance=colder gauge reading, and I'm 99% sure you'll find) will cause exactly what you are seeing.