Silly question, but have you looked inside the radiator? I asked becuse often coolant is checked by looking inside the coolant overflow reservoir. Sometimes the ovrflow reservoir will look like the coolant is normal, when it's low in the radiator.
Here's a link which will give you more info.
http://www.gminsidenews.com/forums/f53/low-coolant-sensor-location-53810/
If your temp guage is not working , there will be a sensor on the engine , often screwed into the thermostat housing which is where the large upper radiator hose connects to the engine.
If you are getting a light that says " low coolant " the sensor is in the bottom of the expansion / overflow bottle.
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