The trouble shows up inside: a stain, a sagging patch, water by a light, or mold. These pages help you tell what it means, what to do first, and whether the roof is the cause.
A ceiling stain is a sign, not the leak: find and stop the water first. Then a few signs on the ceiling itself tell you whether the drywall stays.
A patch under about 10 square feet is in most cases a job you can handle yourself. Cleanup is not finished until the moisture problem is completely fixed.
If water is near a light, fan or wiring, turn off the electricity before assessing the damage.
Mold on a ceiling doesn't show where the water came from, because a damp spot is not automatically a leak: it can come from condensation.
A ceiling stain means water reached the drywall. It does not tell you where the water came from or whether it is still arriving.
Find the source before you cover anything: look from the attic, and leave the closer look at the roof to a professional roofing contractor.
A sagging or bubbling ceiling can be a sign that water is getting in, and you can't tell from the floor how long the drywall will hold.
Frost, damp insulation or drops on the nails without rain can be a roof leak, but they can also be condensation.
Outline the stain lightly in pencil, date it and photograph it. Look at the roof from the ground with binoculars, then check the attic for a drip trail.
Turn off the electricity to that area and leave the fixture alone until an electrician has checked it.