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.

Also verify that the duct cleaner has not damaged the functioning of the systems due to cleaning. Check if the system is functioning properly in both the heating and cooling modes.