HELPFUL TIP #4: How can you tell if it's mold? Usually by the musty odor, because sometimes you can’t see mold. It comes in a variety of colors other than the commonly known dark green, black or brown. You should use a home testing kit, or call in an expert for a home inspection. Besides a musty odor, or the staining, crumbling or swelling of plaster and the separation of baseboards, the following are problem signs of mold: (1) mold and/or mildew; (2) musty odors; and (3) dry rot (dark brown/black fungus on walls).