My heater core went catastrophically at the top of a mountain range in the middle of nowhere. Before that though it had shown symptoms, namely that the plastic baffles at the bottom of the core (like a mini radiator) had flecked apart and disintegrated, by making a sound that I had thought was a leaf in the climate blower like blowing around. This noise had gone on two weeks previous to my disastrous trip.
It is very hard to see if it is leaking without pulling apart some of the carpet. Perhaps you can smell the glyc... stuff of the coolant or get fingers under there and feel for moisture.
Alas, when the heater core goes it can really go.