Since the ducts are not visible unless you actually access them, it is somewhat difficult to asses the situation by yourself. But here are a few signs you need to call your duct pro: 1. If you haven’t change your air filters as required. 2. If the vent covers have visible dust marks. 3. If your AC unit has been working for a long time and the room doesn’t seem to get colder. 4. If the airflow coming out from the vent is low.