Professionally installed shingles should lie flat on your roof. If you see areas that are cracked, damaged, or buckling, you should have your roof inspected to see it needs to be replaced or repaired. Also look for bald spots on the shingles. If you see them it is due to the granules wearing away from the shingle. If you roof has moss or algae, these are also good clues it is time for an inspection. Also, check your gutters and downspouts for shingle granules. If you notice a lot of granules, your roof is awfully close to its useful life.