Water on an upstairs ceiling
Think about what lies above and uphill: roof slope, chimney, valley, roof window or wall junction.
A slate or tile has moved
Photograph it from safe ground. Several disturbed areas justify a wider fixing check.
Water down an outside wall
Watch the gutter in rain and check whether water is running behind the fascia.
Damp beside a chimney
Lead, pointing, masonry and the rear detail all need considering.
A recent repair did not stop the leak
Compare the new weather conditions, mark the stain and find out exactly what was changed.