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How do I know if my home has mold?

Mold is microscopic which cannot be seen with the naked eye. Therefore, it is very hard to determine if your home has mold unless you can visibly see the black growth. Mold needs water to grow and will grow if there are leaks.
Hidden mold can be found by doing some simple sleuthing:
Where do you have water problems?
Check under windows since condensation can form and mold will grow on the sill.
Bathrooms have a lot of water. Any leak or corner that collects moisture is a potential place for mold growth.
The water heater and water softener are notorious for having leaks, especially when the water softener purges. You can often find mold around the edges of their tanks.