There are numerous styles of porous electrodes commercially available that can be used in flow batteries, yet there is no clear consensus as to what styles perform optimally, let alone which physical and chemical properties to select for in future iterations. There are apparent tradeoffs between electrochemically active surface area and electrical conductivity; however, physical heterogeneities leading to mass transport inefficiencies might also produce limited performance. In this study, we investigate the mass transport and electrochemical properties of several commercial porous electrodes in operating aqueous flow batteries using fluorescence microscopy, comparing various figures of merit for their performance. This is enabled using redox-active soluble quinones, which have distinct fluorescence signatures in their oxidized and reduced forms, as a direct measure for in-situ reaction flow mapping. These observations provide insights for future designs of porous electrodes, enabling improved flow battery performance.