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It remains a research challenge in determining the catalytic reaction mechanisms primarily caused by the difficulty to experimentally identify active intermediates with current analytic characterizations. Although computational chemistry has provided an alternative approach to simulate the catalysis process and achieve insights into the reaction pathways, the simulation results would not be conclusive without experimental evidence. Herein, we investigate spatiotemporal electrostatic potential (ESP) distribution surrounding reacting molecules during the catalysis process and suggest its use as a fingerprint to help differentiate and identify active intermediates. Our ESP study of ammonia synthesis on the Ru surface shows a high spatial sensitivity of ESP distribution to molecular configuration and structure of intermediate species and only minor temporal ESP oscillation throughout the lifetime of the intermediates, which provides strong theoretical support to use ESP distribution as a new approach to characterize intermediates. With the ESP measurements at the microscale and in real-time, turning feasible, experimental identification of active intermediates and determination of reaction pathways would become possible by measuring the ESP surrounding the reacting molecules. We suggest developing ESP measurement tools to experimentally explore and unveil reaction mechanisms.