Has the coolant level gone down? There's two tests to check for a leaking head gasket. One involves drawing air from the radiator through a glass cylinder with two chambers partially filled with a special dark blue liquid. If combustion gases are leaking into the cooling system, that liquid will turn bright yellow.
The second test is to add a small bottle of dye to the coolant. After driving for a while, you search with a black light including at the tail pipe. The dye will show up as a bright yellow stain that you follow back to the source.