Redox-active molecules or ions are important in a variety of electrochemical energy storage systems. In lithium-air batteries (LABs), redox active mediators are added as soluble catalysts that mitigate (dis)charge overpotentials as well as promote solution-phase reactions that improve the capacity and cycle life of a cell. Redox flow batteries (RFBs) are dependent on the dissolved species to carry and store charge. In both of these systems, crossover phenomena, whereby the redox-active species in solution diffuse from one side of the cell to the other, result in capacity loss. Here, we report a technique to monitor crossover reactions in lithium-air batteries and redox-flow batteries, exploiting methodology previously developed to monitor radical formation in redox flow batteries. In this technique, radical concentrations are directly quantified operando by flowing an electrolyte solution containing 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) through nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectrometers. We apply this to Li-TEMPO flow batteries and find the coulombic efficiency is only 50%; 50% of the oxidized TEMPO radical, TEMPO+, formed at the cathode, crosses over to the anode where it is reduced, regenerating TEMPO. Numerical modelling simulations of static systems cannot capture the extent of redox shuttling seen experimentally unless extremely fast diffusion of TEMPO and TEMPO+ is assumed in 1D models or convection is included in 2D models, confirming that redox shuttling is enhanced significantly by flow. Finally, we tested Nafion membranes in both flow cells and static LABs and found that the membrane limited crossover of TEMPO and TEMPO+ by factors of ~15x and ~7x respectively.