How do I know if I have mold in my walls?

You usually do not know from the paint alone. Clues are a musty room, paint that bubbles, a stain that returns, or a wall that reads wet on a moisture meter even though it looks clean. Those clues justify opening a small area under containment, not a guess from across the room.

Air samples can support the decision. They cannot see through plaster. In Gettysburg houses we meter first, because a wet stone wall behind new studs is a pattern we already know how to find.