water evaporating from radiator at rapid rate

When checking for coolant loss, I first start with a complete visual inspection. This includes the radiator, engine, heater core area and all the hoses. A pressure test might help indicate a leak also. There also could be an issue with a blown head gasket if there is no traces of external leaking. The coolant has to go somewhere and if it's not leaking externally then I would look to see if it's leaking through the combustion chamber or into the oil pan. I usually use a 4 gas analyzer to check for HC in the radiator to check the head gasket and check the oil dipstick and oil fill cap for signs of coolant contamination (looks like a chocolate milkshake).