Quality control is verifying that work meets a standard before it leaves the business.
The standard has to exist first
A standard that lives in the owner's head cannot be checked against, taught, or applied consistently by anyone else.
Written down as what a completed job of each type looks like, it becomes something a new person can be trained to and an existing one can be measured against.
Checking
For repeatable work, a completion checklist covering the failures that actually occur, drawn from callback history rather than written from scratch.
For larger jobs, an inspection by someone other than the person who did the work. Self-checking catches less, not through dishonesty but because the same assumptions produced the work and the review.
Photographs at completion cost nothing and settle most later disputes about the state work was left in.
What it returns
Fewer callbacks, which is the direct financial return and usually pays for the checking on its own.
Fewer disputes over payment, since documented completion is difficult to argue with.
Better reviews, because the failures that generate complaints are largely the same ones a check catches.
Lower warranty cost — see warranties and guarantees.
Measuring it
Callback rate by job type and by technician, read over a period long enough to mean something. Single months in a small team are noise.
Read by technician it identifies a training need rather than a disciplinary one, which is almost always what it turns out to be.