Active liquid-liquid interfaces, driven far from equilibrium by coupled reactions and transport, govern diverse phe-nomena from synthetic membrane formation to cellular compartmentalization. However, their ultrafast dynamics remain elusive due to the lack of non-invasive probes with sufficient temporal resolution for these buried, molecu-larly thin, and easily perturbed regions. Herein, we proposed a general in situ dielectric sensing approach that tack-les this challenge by transducing interfacial dipole variations into macroscopic capacitance signals, enabling label-free, real-time semiquantitative tracking of interfacial dynamics with sub-millisecond resolution. Applying it to interfacial polymerization —a model system combing interphase diffusion, polycondensation, and phase separa-tion—we directly capture the critical onset of reaction-induced phase separation at approximately 10s, marked by a distinct kinetic inflection. Ex situ model fitting reveals an initial interfacial width of ~1 nm prior to phase separa-tion and a subsequent polymer thickening that follows a power law with an exponent of 0.37. This sub-diffusive scaling signifies a self-limiting growth regime under nanoconfinement, deviating from classical diffusion-limited models. This work establishes a proof-of-concept platform for probing nonequilibrium interfacial phenomena, bridging microscopic kinetics with mesoscale interfacial structural evolution.