The lifetime of Li-ion Batteries (LIB) depends on the reversible Li ion and electron (active Li) uptake and release at the electrodes, ensuring a constant Li inventory. Losses occur for example via parasitic reactions like electrolyte decomposition at the negative electrode, e.g. solid electrolyte interphase (SEI) formation, and are a main reason for capacity degradation. This perspective highlights a simple method on how to quantify these losses at the negative electrode in a simple two-electrode setup with Li metal as the positive electrode. The active Li reservoir of Li metal, can be controlled via advanced charging protocol to mimic the limited Li inventory as found in practical full cells with cathode materials. This method allows fast screening of different mass loadings of the cathode or different state-of-charge (SOC) utilization of the negative electrode, as shown for anode-free and graphite/silicon systems, respectively, which both tend to suffer from active Li losses.