A moldy crawlspace is most often detected by technicians doing service work and by homeowners smells a hard to find musty smell in their homes. Locating mold in a crawlspace can be tricky due to a combination of lowlight and the presence of so many little nooks and crannies for the mold to hide.
Causes are near always attributed to water presence in some form or another. Whether moisture is pooling or condensing is often the concern in determining the source and location of any mold or moldy smells. It is the prevention and repair of the cause of these leaks that is the determining factor in preventing mold or moldy smells in the future. Simply placing plastic over the affected area will only give the moisture another substrate to condense on. This is why many crawlspaces that have been sealed without treating the water issues so often experience rotted wood corroded foundation walls and musty mold smells.