Coolant leaves a white stain when it dries. Should be fairly easy to find a leak unless it's going in your oil.
To check for a bad radiator: locate the plastic-to-metal press fittings on the bottom corners of the radiator. They are an inch or two long and face either side of the car. If your rad is leaking, there will be green coolant pooling where the metal and plastic meet or it will be damp in that area.