Here’s the homeowner trap: the roofer looks and says “that’s chimney work”, the chimney guy says “that’s your roof”, and the stain keeps growing while everyone’s technically not wrong. The intersection genuinely is ambiguous, from the living room. It stops being ambiguous with three observations.
Observation 1: the timing signature
Observation 2: the position
Stains hugging the chimney breast, or ceiling marks within arm’s reach of the masonry: chimney system (flashing first, crown second). Stains offset several feet away, or multiple stains along a ceiling line: roof field, water entered upslope and traveled a rafter before dropping. Water runs along framing before it falls, so offset always means “look uphill”.
Observation 3: the attic verdict
During or right after rain, follow the flashlight: wet at the chimney’s penetration line = flashing. Wet on the chimney’s brick face above the roof line = masonry/crown route. Wet entry at shingles/decking away from the chimney = roofer’s job, and we’ll tell you so for free. The entry point is the verdict; everything downstream is just plumbing.