Most roof problems are cheap to fix if you catch them early and expensive if you don't. You don't need to climb up — a lot can be spotted safely from the ground and from inside the attic.
From the ground
Use binoculars to look for missing, cracked, curling, or buckling shingles, bald spots where granules have worn away, and damaged flashing around chimneys and vents. Granules collecting in gutters are a sign shingles are wearing out.
In the attic
Check the attic for daylight coming through the roof boards, water stains or streaks on the underside of the sheathing, and damp or matted insulation. These show leaks even before they reach your ceiling.
After storms
High winds lift and tear shingles, and hail bruises them. After a big storm, do a ground and attic check. Catching a few damaged shingles early prevents water from getting into the structure.
When to call a roofer
Active leaks, widespread shingle wear, sagging rooflines, or damage you can't safely assess warrant a professional inspection. Many roofers inspect for free and can document storm damage for insurance.