You can easily spot the most visible type of mold, called mildew, which begins as tiny, usually black spots but often grows into larger colonies. It's the black stuff you see in the grout lines in your shower, on damp walls, and outdoors on the surfaces of deck boards and painted siding, especially in damp and shady areas. A mildewed surface is often difficult to distinguish from a dirty one. To test for mildew, simply dab a few drops of household bleach on the blackened area. If it lightens after one to two minutes, you have mildew. If the area remains dark, you probably have dirt.