Parapet walls sometimes tell stories. Dark, damp marks or tide-like stains on them can show where water has regularly pooled or overflowed during rains. If you see water lines at certain heights, it might mean drains got blocked and water rose high before escaping.
Consistent staining in certain corners suggests that those areas are weak points – maybe slopes aren’t right, or drains there clog easily. This can translate into leak risks in adjacent flats or walls.
Even if the roof looks dry when you visit, those older marks show you its behaviour in bad weather.