The best way to check the temperature of your engine is with an infrared temp gauge. Dynamite, Duratrax, Protek R/C, and other brands offer units for less than $30. Place the temp gauge in the center of the head to take a reading. Some temp gauges can protrude into the center of the head and that’s OK, but the temperature reading there will be 10-15°F higher since it’s closer to the combustion chamber.
When checking the temp of your engine, place the gauge at the top of the head and make sure you check the temp at the same place every time.
An infrared temp gauge is an invaluable engine-tuning tool and will set you back only about $30.