Fungal organisms, molds and mildew -- two terms for the same thing -- typically grow in consistently damp locations. When areas inside homes are subjected to excess moisture, mold growth can be a problem. Mold can damage surfaces, and some people may be sensitive to mold spores. This sensitivity may cause respiratory problems or allergic reactions.
The type of mold commonly referred to as "toxic" household mold is a species of black mold called Stachybotrys chartarum. Visible colonies of this mold are shiny and greenish black, sometimes with white edges, when they're wet, and they appear as circular black spots when they're dry. The mold prefers to grow on surfaces that contain cellulose, such as wood, drywall or insulation, and it's unlikely to grow on plastic shower curtains or ceramic tiles. It is not rare in homes, but it is less common than species such as Cladosporium, Penicillium, Aspergillus, or Alternaria.
The best way to know for sure that there's mold in your home is to actually see it. Look in locations where moisture is currently present, where there are stains or other signs of past water damage, or where there is visible evidence of rot or deterioration of surfaces. In inaccessible areas, it may be necessary to remove drywall or other structural materials to perform a visual inspection. In an apartment or other places where demolition isn't an option, look for superficial signs of water damage, such as stains on walls or ceilings, or warped flooring.
Look in these areas for mold colonies, which may manifest as clusters of small dark spots or, in severe cases, large dark areas. Some mold species are lighter orange, brown or green.
Mold colonies often produce a strong musty odor, and the presence of such an odor in your home is a reliable indicator that mold is present. Since mold often grows inside walls and ceilings or in other inaccessible locations, the smell might be the most obvious indication that it's there. Mold inspectors sometimes seal off rooms that are suspected to contain mold and attempt to carefully sniff out the infestation.
Home test kits are designed to detect the presence of mold spores in the air. Typically the kit consists of a dish containing a growing medium that is set out for a period to collect spores, and then the dish is sent to a lab that identifies the mold species. Such tests are limited in their ability to determine the difference between normal and excessive concentrations of mold spores, the types of mold present in the entire home, and the potential health impacts of the mold.