Most roofs do not fail overnight. They give you two or three years of warning first, and the homeowners who catch those signals early almost always pay less than the ones who wait for a ceiling stain.
The signs you can see from the ground
Start from the ground with a pair of binoculars. Most of what matters is visible from the driveway once you know what you are looking at:
Cupped or clawed shingles, which are structural signals rather than cosmetic ones
Bald patches where the granules have washed away
A ridge line that dips or sags in the middle
Granules collecting in your gutters, meaning the surface layer is going
Age matters too. An asphalt roof fitted in the late nineties is past its design life even if it looks passable from the driveway.
What the inside of the house tells you
Not every warning sign is on the roof. Brown rings on a bedroom ceiling, paint lifting away from the top of a wall, or a damp smell in a cupboard that never quite clears are all roof problems that have been running for a while by the time you notice them.
Check the loft on a bright day with the light off. If you can see daylight anywhere other than a vent, water has a way in too.
When a repair is still the right answer
None of this means you need a full replacement tomorrow. Isolated damage on a sound deck is a repair, and we will tell you so.
What changes the answer is water. Once moisture reaches the decking, patching the surface only hides the problem, and every month of delay widens the area that has to come off.