1996 Oldsmobile Delta 88 RADIATOR LOSING FLUID

I DONT HAVE A LEAK IN MY RADIATOR. I SUSPAECT IT TO BE HEATER CORE SINCE ITS NEVER BEEN REPLACED AND THE WATER PUMP HAS BEEN REPLACE IN THE LAST YEAR OR SO. HOW DO YOU KNOW WHICH IS THE PROBLEM. THERE IS NO STEAM OR WATER ON THE INSIDE OF THE CAR YET AS WITH HEATER CORES BUT IT LOST FLUID WITHIN 15 MILES. I SMELL ANITFREEZE AND WHEN I FILLED THE RADIATOR UP IT WAS GONE WITHIN HALF HOUR TO 45 MINUTES AGAIN. IT JUST STARTED THAT THIS MORNING. THERE IS NO RADIATOR LEAK. SO HOW CAN YOU TELL WHICH IS MOST LIKELY THE PROBLEM. JUST HAD A NEW THERMOSTAT PUT ON 2 WEEKS AGO. NO HEAT IN CAR JUST COLD AIR.