Keep a close eye on the coolant level. Remove the radiator cap and check the level when the engine is cold. If you have to keep adding fluid, then you likely have a leak somewhere.

Some parts stores have a radiator pressure tester that they load out. You will pay a deposit and get the money back when you return the tester. Call the parts store before you go, because some stores do not have them. The radiator should be able to hold about 16 psi for a long time (>15 minutes).

 

Another good reason for doing that, is when it's running (hot) the leaking coolant evaporates right away. When you pressurize it cold it can help you find the leak.