I suspect my van's rear heater core has a small leak- I add water often, I don't run hot (unless I allow water to run low), and I do not drop any water on driveway at rear of engine.....!

Question: How to best check out this heater core to check for leaks.....will small seeping leaks be obvious to the eye?  Or is a radiator "pressure test" the way to go w/ this small type leak.

Thanks for insights!

Larry 
Ga.