HOW DO I KNOW WHETHER MY THERMOSTAT IS IN NEED OF BEING REPLACED?

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I have a 2007 Honda accord! I drove my car around all day,running the heater and USED the window defroster!I returned home,park in the garage and turn the engine off! I heard the engine fan running so,I turned the car back on! the fan turned off!I turned the car off again and the fan came on again and I noticed a puddle of radiator fluid on the ground!I allowed the car to cool down for a couple of hours and then started the car again with the heat turned on! the fan blew but it only blew cold air! what could be the problem? the thermostat?