Mold has a distinctive, green, wet smell that is unmistakable once you are familiar with it. As a home experiment, thoroughly wet a towel then wring it out. Roll it up and stick it in a plastic bag allowing air to get in, but not enough for the towel to completely dry out. Within a few days, you will be able to detect the smell of mold on the towel in the bag. Launder the towel in your experiment to restore it to normal.