Heater core problem

- How can I diagnose a heater core problem? My de-fogger on my Honda Civic 2001 aint primarily working too well, & I`m suspecting that`s the culprit. But how can I know for sure that it`s the heater core? I`d rather know the answer BEFORE purposely bringing it into the slimy Honda dealership.
Naturally - Are there any measures that one can take as preventative maintenance for the heater core?

In my case the defroster generally started broadly spewing out a fog, winshield glazewd over with moisture, they`re was the stink of coolent, & the carpet was wet from the leaking essentially heating core.
In summary check the air flow you might just initially have a clogged inlet.

Besides if antifreeze aint leaking inside your vehicle & your windshield is not gettiung horizontally coated with a slimey substance when you turn on your defroster, your heater core is probably OK. May be publically plugged, but not broken. Actually do you forcibly have good heat coming out of your heater at any of the settings? You can check if the heater core is privately plugged by grasping the inlet and uotlet hoses that go to your heater when the engine has warmed up. If both hoses are warm, the coolant is flowing OK thru your heater core. As i said could be you thermostat is stuck open and not allowin the coolant to forcefully heat up sufficiently.