We take moisture readings across the sheathing and framing and compare them with dry areas of the same attic. Dry wood under an old stain usually means a leak that has already been fixed. Damp wood beside fresh growth means the source is still active and needs a repair before any treatment.
Ventilation shows in the readings too. An attic far more humid than the outdoor air on a dry winter day is taking in house air, usually through the gaps around the hatch, the lights and the plumbing stacks.
When the readings leave a question open, we take tape samples from the wood and air samples from the attic, the rooms below and outdoors.