There are ways you can check the thermostat - if you take it out somewhere embossed on the stat is usually a t/stat opening temperature, so you can suspend it in water with a bit of wire and heat it up and using a device to tell you water temperature you could see if its opening at the correct temperature.
But to be honest this is too much hassle and for the cost of a stat you might as well replace it (if you are in doubt) and you anti freeze if it hasn't been done for a while.
I would also suggest that the thermostat should be open, the bottom hose hot, and hot air coming from your interior fan indicating the coolant is circulating correctly before the cooling Fan cuts in when sitting at Idle. The Idea of a thermostat is to maintain a constant running temperature and to get the vehicle to that temperature as soon as possible, and when driving normally the stat is open, the coolant is circulating constantly and getting cooled when going through the radiator.
