Signs of moisture issues under your house can include: damp or musty smells, water stains on ceilings or walls, peeling paint or wallpaper, and rotting wood. If you notice these things, you should get your home inspected as soon as possible to prevent further damage.
There are other ways to check for moisture under your house too. You can use a moisture meter to measure the moisture content of the wood, or you can look for signs of mold or mildew.
It is also important to keep any areas around your home, such as cellars, basements, and crawl spaces, dry and well ventilated. This helps prevent mold and mildew growth.