Look in the gutters
Asphalt shingles are protected by a layer of mineral granules. When those wash off in quantity and pile up in the gutters, the mat underneath is exposed to the sun and the clock speeds up.
Look at the field from the ground
Curled, cupped, or cracked tabs across the whole roof are age, not damage. A few lifted shingles in one spot after a storm are damage, and damage is repairable.
Look inside
From the attic, daylight through the deck, dark staining across the sheathing, or soft wood are all signs the problem is no longer on the surface.
The honest test
Ask how many separate repairs the roof has needed in the last few years. If the answer is more than a couple and the covering is old, money spent patching is money spent twice.