While some mold growth is pretty obvious, you have to know where to look to find several of mold's favorite hiding places. These include closets that have exterior walls (walls that face the exterior of the building), and on the unfinished, wooden backs and bottoms of furniture that has been stored in damp spaces.
Sometimes, shining a flashlight sideways along a surface will reveal the fuzzy, often circular patterns of mold growth. Be careful, however, not to move possessions around as you search for mold, as this will disturb any mold present and aerosolize (make airborne) the mold spores.
A musty small can also sometime mean mold is present. In any event, an air quality professional can best help you find mold sources, and remove mold and its musty odor.