Beyond the cell design, we proposed and validated new diagnostic tools for CO2 electrolyzers to deconvolute catalytic and non-catalytic CO2 consumption. We defined an experimentally accessible Carbon-Crossover-Coefficient (CCC) as the ratio between non catalytic CO2 consumption and generated alkalinity. The CCC values of 0,1, and 2 were associated with the three physically limiting conditions of pure OH -, CO3 2-, and HCO3 2-transport. We also showed how CCC-FECO maps define regions of distinct in-plane and through-plane transport regimes.