Are the shingle edges curled up or cupped? Are they cracked? Is cracking or cupping especially noticeable in the valleys on the roof? This is an indication the shingles are starting to fail and failure in the valley areas is a sure sign you are going to have leaking. The shingles in the valley areas guide the water flow down to the gutters. You want water in the gutters, not in the attic!
Do the shingles lift up in certain areas when the wind blows? Are there shingles missing? Is the gutter filled with granules from the shingles? Are there streaks of dirt and algae on the roof? These can be cleaned off but scrubbing them means you will be taking some of the granules from the shingles off, too and compromising the integrity of the roof.
Is there moss growing on the roof? Some people think moss on the roof adds character and charm. It also adds a layer of vegetation that holds moisture on the roof and over time with freezing and thawing will crack and split the shingles.