How do you determine if the heater core is bad?

check for leaks, usually wet in the interior front. do or have a pressure test done on the system, use the color dye to identify the location. If that is not the problem then that leave 2 more to try. the problem may be in the venting of the air. sometimes the vent controls don't work.(redirect the air) They can get stuck. they have three main methods of control. electical, mechanical, or vaccum. that is what i can think of for the minute. hope it helps. duboff it will leak fluid. however they can become plugged if you have failed to change your antifreeze for a long time. this would restricting the flow throw it. If your radiator is gummed up odds are the heater core is also... only check while the engines cold!