Visible Mold - Mold can appear threadlike and white, or it can look like dirt. Even a tiny amount can signal a more extensive out-of-sight infestation. Odors - Mold has a distinctive musty scent. The smell is strongest in rooms that have a mold infestation. If you only smell the scent when your heating unit or AC is on, the mold infestation is probably in that system. Health Issues - If you have nasal congestion, a running nose, or itchy eyes, you may be experiencing an allergic reaction triggered by mold, particularly if symptoms worsen when you're in the house. If you have nasal congestion, a running nose, or itchy eyes, you may be experiencing an allergic reaction triggered by mold, particularly if symptoms worsen when you're in the house.