If you discover that your roof is leaking—most likely evident by water stains on the ceiling—note the leak’s location.  Look to see if there are any curled, cracked, or missing shingles.  Leaks can occur at any point where shingles butt, or where caulking and flashing have been compromised.

End caps, the tent-shaped shingles that cover the angular peaks of the roof, can also be the source of leaks, so check those as well. If you discover shingle problems (which you can do from the ground with a good pair of binoculars), repairs may be an easy fix.  Curled-back shingles, for example, can be re-secured by brushing on a coating of asphalt roofing cement or finding its equivalent compound in tubes for use with a caulk gun.  You’ll want to apply a generous amount of roofing cement to the underside of the shingle to make certain that the edge and corners are secured.  Then press firmly to set.

Source:  http://www.bobvila.com/articles/roof-leaking-repair/#.U45cRCiiX94