We demonstrate an environmentally friendly and easily handling cold flow diagnostics to determine the value of exhaust velocity c∗ in bi-propellant thrusters, practically useful prior to the hot firing tests. We employ only water as the simulant liquid, and directly measure the local ratios of mixture and flow rate using a conventional patternator with an absorbance spectrometer. The density ratio mismatching to the real propellants of MON/MMH is able to be reasonably compensated. Combining with the chemical equilibrium analysis, we calculate c∗ at wide range of injection mixture ratio under several injector configurations. The validity of the framework is quantitatively evidenced by comparing with corresponding hot firing tests. This fact deduces that the mixing states of MON/MMH at firing condition is reproducible by water flow diagnostics under the appropriate conversion technique. We also investigate the c∗ trend resulting from injection conditions by flow visualizations.