How can I tell if the problem is the heater core or the water pump?

The problem is: I am filling up my antifreeze resevoir every other day, heat is not working very well, the motor fans come on with in a mile or so of driving (this is not normal, so, I have NOT been turning the heat on (as much) and it seems the fans haven't come on as frequently). Recently, in the past couple of weeks, the temp gauge has shot into the HOT zone (turning the heat on or off seems to alleviate this, phew!). I had the car pressure tested a couple of months ago, and nothing appears to leak nor is there any evidence of leaking under car near radiator or anti freeze resevoir. AND I use way more gas (at least in the winter). That's my story.

Hands-on testing and visual inspection along with pressure test and perhaps a combustion leak test is about the only way to find where the coolant is going... IF it were the heater core leaking.. Passenger side carpet should be soaking wet along with a badly fogged up windshield plus antifreeze smell.. Sometimes, mysterious loss of engine coolant can be difficult to diagnose... Good luck with it!

Thank you!!