If the roof leaks the house will be damaged. Look at it from the ground or out the windows. Walking the roof is dangerous and may damage the roofing. If you can't see it, be suspicious. Asphalt shingles cupping or curling, and missing shingles or slates tell you the roof is aging or damaged. Tar on the roof or flashings may mean poor workmanship on repairs. Rust on metal roofing or flashings is not good. If the paint on the metal is flaking or peeling it is failing. Look for stained or rotted wood in the attic and stains on ceilings. These stains indicate either an active leak or one that has been possibly repaired.