Have it pressure tested if you can't see it otherwise but the first place you should now look is inside the car. Remove the plastic cover under the carpet to the right of your gas pedal and look at the heater core area and pipes between the firewall and heater core. Check under the matting below the carpet there too for signs of coolant.
If it's dry, then a pressure test should be able to locate to source and if internal engine leakage is suspected, remove spark plugs while it's under pressure and see if coolant's getting inside the cylinders.