Compare an indoor surface temperature with the estimated dew point.
For ordinary indoor conditions; not a frost-point calculation.
Use a humidity measurement from the same room and time.
Consider measurement error, especially on reflective surfaces.
Estimated dew point
12.0°C
Surface minus dew point
-2.0°C
Magnus approximation over liquid water. A negative margin means the surface is below estimated dew point. A positive margin does not rule out mold or hidden moisture. This is not an interstitial-condensation model.