Roof shape and old repairs
Uneven courses, many replacement slates, large patches and altered roof junctions are worth asking about. They are clues, not a diagnosis.
Loft access
During a proper survey, fresh damp, soft timber, damaged underlay and poorly altered rafters matter more than harmless old staining.
Extensions and roof windows
Every new junction adds detail. Ask when alterations were made and whether leaks have occurred around them.
Think about access
A defect beside a high chimney can cost much more to reach than one near a low eaves. The practical cost of access belongs in your assessment.