Missing, curled, or cracked shingles are the easiest sign to spot. Walk around the house with binoculars after a windy day. Any gap in the pattern is a spot where water can reach the decking.
Granules in the gutters mean shingles are wearing out. A small amount is normal on a new roof, but handfuls of grit in the downspouts on an older roof means the protective layer is going.
Look up at the flashing around your chimney, vent pipes, and skylights. Rusted, lifted, or caulk-patched flashing is the most common source of leaks we repair.
In the attic, check for daylight through the boards, dark stains on the wood, and damp insulation. A sagging roofline seen from the street is the most serious sign of all, because it usually means the decking underneath has been wet for a long time.
If you see any of these, get a free inspection before the snow flies. A repair in October costs far less than one in January, and it costs much less than a ceiling.