if your loosing coolant externally you will find a puddle under the car.... you may try to have it idle for 5 - 10 mins to see if one forms.
I would also check your oil to see if its milky or over filled.... this is usually a sign that the coolant is getting into your oil.
If you dont see anything on the ground then you most likely have an internal leak.
If you still feel like you have an external leak you can pick up coolant dye and a black light. Put the dye in, wait till dark or put it in the garage with the lights off and shine the black light around. You will find a glowing spot.
If you think its most likely an internal leak. The exhaust will usually smell sweet and/or produce steam.
You can most likely borrow a compression testing tool at some parts stores for a small fee. You can also buy one at most places that sell automotive tools. That will tell you if you have coolant leaking into your cylinders and which ones it is.
You can also borrow or rent a coolant pressure tester, that will tell you if in fact you have a leak. But it wont tell you where its leaking unless its external.
