ideally you weigh the refrigerant charge. Really experienced guys can tune the system with a few tools like a gauge manifold, thermometers and some calculations.
Depends on the level of skill of the person handling the job.
Honestly they can just hook up a manifold and see the run pressures and output temperatures to actually "see" if the A/C is upto mark. Their weight thing will be invalid if there is a failure in the unit like a bad expansion valve.