With the engine cold, remove the radiator cap and start the car. Watch
for bubbles to appear in the coolant, particularly after the thermostat
opens. If you see them, it's the head gasket. You may also be able to
smell antifreeze in the exhaust.

Gary

Look closely at the side of the block below the water pump. It probably
just leaks cold and not enough to drip very noticeably on the ground.
When they leak very slowly the coolant runs along the oil pan gasket and
drips from the bell housing. It self seals when warm and driving
evaporates the small amount of coolant as you drive. There will be a
crystalline residue on the motor below the pump and perhaps a line of
dried coolant on the bottom of the hood from the fine spray coming off
the fan blades.

Bob