Cheapest solution:
As Steve suggested, if you haven't replaced them, check the hoses for leaks. Don't overlook the heater hoses. They can leak and you might never see it. Also, check the overflow tank and its cap for leaks and cracks.

Does the car run well otherwise or does it run/idle rough or poorly? Normally, a car with a bad head gasket will give you some sort of clue.

Any antifreeze smell inside the car? That would point to a leaky heater core.