What genuinely indicates it
Camera footage showing a collapse, a separated joint, or a section that has deteriorated along its length. Repeated failures at one specific point. A line that fails again immediately after being cleared properly.
What does not indicate it
A drain that clogs repeatedly. That is the reason most often given for a replacement quote, and on its own it is not evidence of anything structural. A sound pipe with a coated wall will block again and again until the wall is cleaned.
The question to ask
Ask to see the specific fault on camera, and ask what would happen if the line were cleaned properly instead. A reasonable answer to that second question exists in both directions, and you are entitled to hear it.
Getting a second opinion
On a five-figure quote, a second look is cheap. Any company confident in its diagnosis should be comfortable with you getting one.
Why this gets oversold
Replacement is the highest-value job in this trade, and a recurring blockage is an easy thing to present as evidence of one. The gap between what a symptom proves and what it gets used to justify is where most of the money in this industry goes.
What a fair recommendation looks like
It names the specific fault, shows it to you, gives its distance from the house, and explains what would happen if the line were cleaned properly instead. If any of those is missing, ask for it.
The middle ground people forget
Between cleaning and full replacement there is spot repair and lining. A single failed section does not require the whole lateral to be replaced, and being offered only the two extremes is itself a reason to ask more questions.
What to do with a quote you are unsure about
Ask for the footage and the distance to the fault. Then get a second opinion from somebody who has not already quoted you. A company confident in its diagnosis will have no difficulty with that.