- Water spilling over the front edge during heavy rain rather than reaching the downpipe.
- A downpipe that stays dry while the gutter above it is clearly full.
- A vertical stain or a green streak on the wall below one point in the run.
- Plants growing out of the gutter, which means there is enough silt in it to hold roots.
- Water running behind the gutter and marking the fascia rather than falling clear.
The last one is the one worth acting on quickly, because what it wets is timber rather than masonry. If the roof above is also shedding moss into the run, the two jobs are worth doing together — roof cleaning puts everything it removes into the gutter, so the order matters.