Thermostat

Check to see if fluid is actually flowing, it may be the pump that is bad or restriction somewhere

How do I do that.....

First, check to see if cooling fans are turning on when engine starts to get hot, if they are, when engine cools down, remove radiator cap, start car up and see if u see water flowing or u can actually pinch the upper radiator hose and feel the water flowing. Pump sucks coolant from bottom of radiator and pumps it back in on top