Moisture in an attic can build over time.  It can take quite a while before it makes its way down through the ceiling and onto a light fixture (for example).  Take a look into your attic every once in a while to ensure there is no moisture.  One of the first signs when you open the hatch and look up is the underside of the roof.  If you have a wet attic roof you have a problem that must be addressed right away.  If things seem to be Ok but you can still identify black staining on the wooden sheathing of your roof (the roof deck composed of wood slats, plywood or OSB) you may have an indication of a prior wet attic roof.