Shingles that are cracked or curled: A normal sign of wear and tear, but it could also be a manufacturer defect that results in the early declination of the shingles integrity.
Finding shingle granules in gutter: another sign of normal wear and tear
Moss or algae growing on your roof: moss can push under your shingles forcing them up, which can lead to thin spots and holes in your roof which eventually result in leaks
Leaks and water damage: If water has made its way inside your home, there is a good possibility that your roof and roof decking has been compromised and need to be replaced before it causes further damage to the interior of your home.
Your roof is more than 20 years old: Roofs usually only have about a 20 year life span(sometimes less). If you know your roof is approaching that age, it's time to start planning for a roof replacement.