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.

Always ask for references from the duct cleaner. Only appoint him after you are assured that other customers were satisfied and did not experience any complications with their cooling or heating system after cleaning.