this pertains to a 1998 Honda Civic LX, a/t, a/c, 230,000 miles

Over the last 6 or 7 months i find that every 3 or 4 weeks i have to
add 1 or 2 cups of coolant to the overflow container in order to bring
it up the MAX level. Several thorough visual inspections of the
engine and of the engine compartment have failed to locate any leaks,
and there are never any puddles or even drips of the stuff on my
garage floor. Where can the coolant be disappearing to? Could it be
leaking head gasket? and if so, are there any specific tests i can
perform to confirm that diagnosis, rather than just replace the gasket
and hope that that will fix the problem? What else might account for
the missing coolant? (The engine on this car never runs hot.)
Thanks in advance