The first and perhaps most obvious place to look for a roof leak is directly above the leak in a ceiling or exterior wall. Use a flashlight to inspect the attic floor over the leak while it's raining. Look for standing water, water stains, mold, wet insulation or other exposed insulation.
Examine the underside of the roof for wetness or mold around points of penetration (plumbing vents, chimneys), wherever different roof planes intersect (valleys) and near dormers. These symptoms indicate holes in the flashing or faulty flashing installation.
A leak away from such locations suggests a problem in the roofing material. Keep in mind that water may travel sideways before passing through a joint in the roof sheathing, and may travel in a horizontal joint before falling on the floor or ceiling.
Take measurements from points inside that you can also locate from outside. Measure down from a ridge and horizontally from the center of a valley or sidewall; or measure distances from a chimney or other point of penetration.
If your ceiling is attached to roof rafters, as would be the case for a cathedral ceiling, all you can do from inside is take the measurements that will help you locate the leak externally, and attempt to control the damage internally.
For further details: Roof Leaks Repair