Although boronic acids are widely used in metal-catalyzed reactions, it is difficult to assay their consumption. As such, we developed a reversible fluorescent sensor that is activated upon binding a boronic acid. The sensor can be used to monitor consumption of a boronic acid in Suzuki-Miyaura reactions. Importantly, only a standard handheld long-wave UV lamp (365 nm) is required and fluorescence is easily detectable with the naked eye without disturbing the reaction mixture.