<p>Hidden catalysis occurs when an impurity or species generated in situ facilitates the reaction instead of the intended catalyst. This pervasive issue plagues reaction development and fundamental understanding, with prominent examples including trace metal contamination, ‘metal-free’ reactivity, hidden (Brønsted) acid catalysis and hidden borane catalysis. Current methods to identify hidden catalysis are hindered by time-consuming and labour-intensive mechanistic analyses, limiting their practicality and widespread adoption. We introduce a transformative, colorimetric indicator that enables rapid, visual detection of hidden borane catalysis. The colorimetric test was successfully used for reaction sampling, in situ testing, reagent quality control and even test strip analyses. This rapid, visual detection method removes the necessity for laborious and costly mechanistic analyses and offers a powerful tool for chemists to quickly identify and mitigate hidden catalytic effects. This method holds the potential to substantially accelerate discovery and optimization in chemical synthesis by clarifying mechanistic understanding at the outset.</p><p></p>