Verify if the duct cleaner installs access doors in sheet metal ducts attached with more than just duct tape, e.g., screws, rivets, mastic, etc.

Verify if the duct cleaner checks whether there is air leakage through access doors, which may be slight or non-existent. Such air leakage can damage the duct.

See that the duct cleaner checks whether all air vents have registers, grilles, and diffusers. He must also see that they are firmly re-attached to the walls, floors, and/or ceilings.