You can check the level through the overflow tank shown on the check fluids page. Careful if the engine is hot? Best wait until it is cool. Leave the hood up to cool faster. Later try removing the cap but SLOWLY and if there are no steam coming out.
The overheat problem I had was related to a coolant temp sensor, on the 3.1 it is on the driver's side. Under the throttle body control. Replaced it and the fans don't run until the engine is hot.
When the engine now heats up on mine, the car still overheats and because the engine is hot and the radiator is cool gives a clue. If there has been coolant loss and a refill, it might be due to air in the coolant lines, and so don't be so quick to guess it is the thermostat. On the 3.1 they are a b....h to change. But hope that give you ideas. Also google for the replacement of such on google u tube and watch the movies. That is good. OK? Hope this helps.