Accurate, real-time detection of dew point of a gaseous sample can be accomplished using the systems and techniques described herein. A gaseous sampling chamber defining an interior volume includes a patterned structure having a roughened surface exposed to the gaseous sampling chamber. The patterned structure includes an open volume accessible by the roughened surface, for example, representing at least about 10% of the interior volume of the gaseous sampling chamber. An illumination source is configured to illuminate at least a portion of the patterned structure. A light detector is configured to receive at least a portion of illumination returned from the patterned structure. A condensate detector is configured to determine a presence of a condensate on the roughened surface in response to an optical property of the patterned surface as modified by the presence of dew.