Best way to know is, Open the hood, Let your car idle, When you see the temperature gauge going past the half way mark, and starting to overheat,Look at the radiator fans, If the car is starting to overheat and the radiator fan does not turn on, you have an issue with the radiator fan system. If the fan comes on, use your hand, carefully grab the upper radiator hose and feel its temperature through you hand. It should be really hot. Now grab the lower hose, the temperature should be the same. If one is a lot cooler than the other, you may have a faulty thermostat.