Water on an upstairs ceiling
Think about what lies above and uphill: open roof slope, chimney, valley or roof window.
A slate or tile has moved
Photograph it from safe ground and compare the surrounding courses.
Water down an outside wall
Watch the gutter in rain and check whether water is getting behind the fascia.
Damp beside a chimney
Lead, pointing, masonry and the rear detail all belong in the inspection.
A recent repair did not solve it
Compare the weather conditions and identify exactly what was changed.