Watch it in the rain if you can
You can learn more from ten minutes of safe observation during rainfall than from staring at a dry gutter the next morning. Look for water spilling over the front, running down behind the gutter or escaping at a joint.
Blockages are common but not the only cause
Moss, leaves and roof grit collect around outlets. Brackets can also sag, leaving a low point where water sits. On older metal gutters, joints may open as fixings move.
Downpipes matter
A clear gutter can still overflow if the downpipe or shoe is blocked. If water disappears into a hopper or gully, check that the next stage of the drainage route is actually taking it away.
Wall damp below the eaves
Before anyone starts lifting tiles above a damp patch, look at the gutter directly outside. It is a basic check, but it saves unnecessary roof work more often than people think.