You buy a water temp gauge and a sending unit which screws in the engine somewhere it can sense the water temp. You can drill out a hole and tap it to install the sending unit in the thermostat housing.
There is a water temp. sensor more than likely on the radiator or on the manifold where the water goes to the engine from the radiator. AutoZone will usually be able to tell you where switch is located. It may be the fan motor. Turn on the a/c the fan should run .