What this video covers
The tool in this clip costs about thirty dollars and is a digital hygrometer, which measures relative humidity.
The recommendation is to place several rather than one: basement, bathroom and bedroom, because humidity is not uniform across a house. The basement can sit well above the rest of the building without anyone noticing.
The takeaway
Humidity is the one mold input you can measure directly and cheaply. A number on a display turns a vague sense that somewhere feels damp into something you can act on before growth starts.
Read the full written guide: The $30 Tool That Prevents Mold.