here's how you can tell if its the thermostat or not:
-if when you drive the car, at highway speed or ~55 - 60mph, and your engine temp goes down, then you are probably having a problem with the cooling fan. it cant be the thermostat, because the thermostat is obviously letting fluid flow through the system normally, all you needed was the extra air being forced through the radiator to cool down the fluid inside which would normally happen if your fan was coming on at a complete stop.
-if your thermo. is stuck closed, then the engine will overheat even if the fans are coming on, because even though the fluid in the radiator is cooled, its not getting back to the engine block/head to actually chill it down. What you should do to confirm this, is turn your heat on full blast, and if the engine's temp gauge goes down then your thermo. is most likely stuck closed.