The answer
Water travels along rafters and vapor barriers before dripping through drywall at the lowest depression. Inspect the attic with a flashlight above interior stains, tracing water marks upward. Measure to the ridge/eave and inspect corresponding exterior roof penetrations (flashing, chimney, valleys, plumbing vent boots, skylights). Certified inspectors use ASTM spray tests or infrared thermography.
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.