What is mold?
Mold is a type of fungus, which can be found almost anywhere; grow virtually on any substance, providing moisture is present. There can be potential health effects & symptoms associated with exposure. Even if you cannot see mold, it doesn't mean there is none there, it can grow on the backsides of walls, ceilings, floors or in ductwork.
Some common reasons to suspect a mold issue is: it's visible, notice a musty smell, have allergic symptoms or have (had) water problems. https://www.epa.gov/radon