Curled or cupped shingle edges across whole slopes.
Bald spots where the granules have worn off and the asphalt is exposed.
Granule buildup in the gutters after a rain.
Nail heads backing out and showing above the shingle surface.
Daylight visible through the roof deck from inside the attic.
Sagging between rafters, which is a structural issue and not just a roofing one.
You are patching leaks in more than one location a year.