Abstract <p>Rubber vulcanization is a crucial step in manufacturing, significantly impacting material performance. Traditional methods, relying on physical properties or free sulfur analysis, struggle to accurately assess vulcanization completeness due to the presence of intermediate sulfur species (ISS). ISS are critical indicators of vulcanization completion, but their specific detection remains challenging. This work introduces a simple, and visually intuitive method for identifying ISS in under-cured rubber. The method involves immersing rubber fragments in a colorless Au³⁺-containing alkaline solution. The presence of ISS in the rubber leads to a distinctive yellow coloration of the solution. This unprecedented colorimetric reaction was validated using a complex rubber system employed in commercial train lateral stops. The observed color change was specifically attributable to ISS, not the raw materials or the fully vulcanized product (bridge sulfur). This method also detected ISS in various commercial rubber products, suggesting potential incomplete vulcanization in these materials. This chemical assay provides a valuable tool for assessing rubber vulcanization quality, guiding the optimization of formulations and manufacturing processes to ensure complete reduced levels of specific ISS detectable by this method and maximize product performance.</p>