Im assuming a late model here..
There is a pair of hoses coming from the firewall back in the recess where it turns into the tranmission tunnel. These hoses will come to the front of the front of the engine, or atleast one of them will go to the water neck area(havent messed with any V8 models).
Anyhow, what I did with my Ranger was I disconnected both lines, and I plugged the end of one with something, anything. And then the other one I jammed an airline fitting for my air compressor. I then proceeded to put about 25psi into the heater core with the regulator. Also, usually if your core is leaking the window will fog up when you turn on the heat, and you can smell the coolant(as long as you are using conventional coolants).