Solution for a Small Leak
Some roof leaks are tough to locate. Sometimes the water shows up at a ceiling spot distant from the leak. If your ceiling has a plastic vapor barrier between the drywall and the attic insulation, push the insulation aside and look for flow stains on the plastic. Often water runs to openings in the vapor barrier, such as at ceiling light fixtures.
If you can't see any telltale flow marks, and since the stain is fairly small, look at the underside of the roof for 'shiners.' A shiner is a nail that missed the framing member, in this case when the carpenter nailed the roof sheathing to the rafters. Moisture that escapes into the cold attic from the rooms below often condenses on cold nails. Sometimes you can spot this if you climb up into your attic on a cold night. The nails will look white because they're frosted. When the attic heats up a bit during the day, the frost melts and drips, then the nails frost up at night again and so on. The solution is to simply clip the nail with a side-cutting pliers.
Fix Roof Vents
Metal flashing prevents leaks where shingles meet other surfaces, like walls and chimneys. Proper flashing work takes time and know-how, so sloppy roofers sometimes slather on roof cement instead. It seals out water long enough for them to cash your check, but it soon hardens, cracks and leaks. In the end, all it does is make a proper repair more difficult. So if you see heavy 'tar' patchwork on your roof, fix it right—before it leaks and leads to interior damage.