Hygrometer on Amazon. I think they are two for $12.

leave one in there and pay attention to the humidity level during rainy days and in humid summer.

Maybe scrub off what you can reach with soap.

If you are worried about it still. Either spray bleach water in there from the top down and hope to shock the mold.spores that could float around the house. Or get ozone blasted in there to kill the spores. Unless you have a cooky friend with an ozone machine like me you probably need a pro for that.

There's also one non chemical option . It's called benefect and it's mostly essential oils. I hired a pro to apply it in a clients house once. Way safer than the hard stuff. GL