I forgot that modern cars don't have little shutoff valves on the heater core feed hose. I guess I was thinking of the old Triumph TR-3 we had when I was growing up. Under the hood it had what looked like a garden-variety faucet handle you had to open to get hot water to the heater core. If the water is flowing constantly to the core, then I guess the best way to check that is feel the return line. But, Z3bra, it sounds like you might be running kind of cool anyway, unless it's just the gauge.