The mold survives and flourishes on any surface that holds a moisture film. If moisture accumulates faster than the natural drying out process, the mold is in a flourishing environment. Such an environment promotes rapid amplification, that is, a colonization and growth of the fungi. Some of these biological organism can then emit spores containing mycotoxins and mVOC, microbial volatile organic compounds. 

Myotoxins are a metabolite. They are toxic to humans and animals in low doses, although the dose-response levels are not yet known. mVOC are allergens and are evidenced by a musty odor. 

This musty odor is evidence that mold is growing. Scientific evidence is obtained through bulk air samples of particulates that are viable-- that is, can be cultured in a laboratory. Evidence of the existence, but not necessarily growth of mold, is through visual inspection. Scientific evidence of non-living mold contaminants in the air is obtained through bulk sampling of non-viable particulates.