Record the pattern rather than testing it yourself.
Explain if water comes over a length of gutter, spills at one outlet or drips from a connection. Tell us about visible leaves, plant growth or earlier debris removal. Fallen moss fragments can be loose material in a channel; they do not turn gutter clearance into roof treatment.
Do not pour water from an elevated position, reach into an outlet or dismantle a pipe to find the cause. Previous photographs and written observations help without creating another hazard. A mark on the wall alone cannot confirm the present condition of the gutter.