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.
Quality assurance and quality control are often used interchangeably and are not quite the same thing. Quality control is checking the work after it is done — inspection, and catching what went wrong. Quality assurance is the arrangement that stops it going wrong in the first place: the written standard, the training, the checklist a technician works from. Control finds the callback; assurance is why there were fewer to find. A business relying entirely on control is inspecting its way to quality, which is the more expensive of the two.
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.
Writing down how a recurring task is performed, so it can be delegated and improved, is a standard operating procedure.