The easiest form of mould that can be spotted is the most visible type of mould, called mildew, which are black spots at first, but then they often grow into larger colonies of mould. It’s the black stuff you see in the lines of your shower, outdoors, on damp walls, and on the surfaces of deck boards and painted siding, especially in the shaded, damp areas. A surface that is suffering from mildew is often difficult to distinguish from a dirty one. A way to test if it is mildew rather than just dirt, dab a few drops of bleach to the area. If the area becomes lighter after one to two minutes, it is mildew, if not, it is just dirt.