The ways to check the head gasket without stripping it apart are:
Run the engine until warm, stop it, take all the spark plugs out and check each one in turn to see if there are any rusty deposits on it or if it looks unusually clean compared to the others.
Leave the plugs out for an hour then get someone to crank the engine over and see if any water comes out of the plug holes.
After that, a compression test on each cylinder to establish if one is leaking out. You can also pressure test the water system to see if it's losing pressure.
Water pumps sometimes only show signs of losing water when the engine is stationary after it has reached temperature as the water is forced past the bearing seals and forms a drip.
As said a few times, check the footwells for sticky damp patches which will indicate a leaking heater matrix.
If all else fails and everything looks right - when is the last time the water level was checked? It could just be a tiny leak that has been there for a while but just got to the point where it's noticeable, ie not enough fluid in the system to keep the heater matrix from air locking.