Look underneath the vehicle and see where there is fluorescent green liquid dripping and then follow the flow back to its source. Remember it will follow the flow of gravity and will also wrap around objects.
Special tools are available for around $80 dollars that allow you to simulate a hot engine without the risk of being burnt. These tools are like bicycle pumps that attach to the radiator in place of the radiator cap and allow you to pressurize the cooling system as it would when the engine is hot. These tools have a gage on them and the pressure on the gage should not exceed the pressure of the radiator cap. Once pressurized you can look around and see if the Coolant Liquid is coming out. If you are lucky you should see an obvious stream of coolant or constant drip at the source of the Coolant Leak.
When you pull out your oil dipstick does the oil look milky? When water mixes with oil an emulsification effect happens making the oil more gooey and milky in color. If you see this you have found that your coolant leak as it is leaking inside your engine.
With a cold engine that is NOT running. Put on a disposable glove and feel in areas that you cannot see. Pull the hand out and see if you can see any fluid on it.
Radiator Fluid (Coolant) has a very distinctive smell. If you smell this inside your car then you probably have a coolant leak in there. This means that either the heater in the car of one of the hoses to the heater is leaking coolant.