How the result is decided
The rules are simple and fixed. Sewage coming up inside right now means act now. Neighbors backing up or sewage at a street manhole points to the City main. Otherwise, signs that point to the shared line add up: several slow drains on different sides of the house, repeated clearing, a wet or smelly spot along the line, gurgling, or a camera report that already shows a defect. Enough of them together means schedule an inspection soon.
One slow fixture on its own usually means a clog in that fixture or its branch rather than the sewer line, which is a cheaper fix. That distinction matters, because a single clog is sometimes sold as a failing lateral.
What it cannot do
It cannot see inside the pipe. Roots, offsets, a belly or a collapse only show up on camera, and the right repair depends on which one it is. The camera inspection page explains what happens on that visit, and signs of a broken sewer line goes deeper on what each symptom can mean.