Watch the gutter in rain
A blocked outlet can back water up for several metres. Look for the point where the normal flow stops.
Check the fall after cleaning
Debris removal will not correct a gutter that sags between brackets. Standing water after the rain has stopped usually tells you the level is wrong.
Look behind the gutter
Water can run down the back of a fascia without dramatic front overflow. Repeated wetting there can damage timber and masonry.