I'm getting my temperature guage fluctuating from high to normal but my engine is not overheating .What should I do ?

Hello, Not to insult your intelligence, but how do you know the engine is not overheating? The fluctuations tell me it is likely a bad thermostat, coolant temperature sensor, or something else depending on when the fluctuations occur, what makes it go up, and what makes it go down. Also, did it start after snowing or freezing weather?