Hi. If you are wondering how hvac mold testing. Molds are usually easy to see. However, while they are invisible, you will notice their stench in the air or certain symptoms in family members or pets. Most of the time you can smell mold in your air ducts or vents before you can see it because of the stench. As a result, if your heating and cooling units (if they share air ducts) begin to emit a musty or musty smell anytime you're not using your air conditioner, mold is likely growing in your HVAC system.