That and you should also be able to watch the temp gauge and tell. For example (assuming you have aftermarket gauges) the temp starts to rise and then once it gets to the thermostat temp (IE 180 degrees, 195, etc) it pauses and or holds at that temp (like so)http://www.stangnet.com/mustang-forums/8257208-post12.html. This is assuming of course that the rest of your cooling system is functioning properly. Or better yet, pull the stat and heat it up in a pot of water while watching it with a thermometer. What is your fan doing now?