Pressure test the cooling system. Look at what the pressure is rated at on the radiator cap, pump it up to that psi. If it holds, it's probably not a head gasket. If it doesn't hold, check for an external leak somewhere. If you don't find one, it's probably a head gasket.
OTOH, it could be exactly what Casey said.
Check and see if you've lost any coolant, could be another sign.