Checkups should be done every spring and every fall. The sun does more damage than the wind and rain so pay close attention to the sunny side of the house. The first place to check is along the ridges of the roof, the points where the shingles are folded over. Any cracks you see can easily turn into leaks if not caught in time. Check the valleys next.
Valleys are joints that have a downward slope.
Most are covered with a piece of sheet metal called flashing. Make sure there are no holes or rust spots. You may see more flashing around any structures that rise out of the roof like chimneys. Check this for imperfections as well. Next look for missing, curled or loose shingles.