Wet ceiling after rain
Check what sits above and uphill from the mark. Open roof slope, chimney, valley, roof window and wall junction each lead to different inspections.
One tile or slate out of line
Photograph it from the ground. One movement can be a local fixing problem. Several disturbed pieces suggest checking the surrounding course and fixings.
Water down an outside wall
Watch the gutter during rain. Look for overflow, leaking joints and water running behind the fascia. Then inspect roof-to-wall details above if relevant.
Damp around the chimney
Consider leadwork, pointing, the rear gutter or soakers, flaunching and the flue itself. Ground-level views rarely show all of these.
Leak after someone already repaired it
Compare the new weather conditions with the old leak and find out exactly what was changed. There may be a second route nearby.