Relaxor ferroelectrics exhibit extensive applications in sensing technology, optoelectronics, high-density memory storage, and neuromorphic computing, owing to their superior dielectric and piezoelectric characteristics. However, conventional methods, including the Sawyer-Tower circuit and the positive-up-negative-down (PUND) pulse train, prove inadequate for nanoscale ultra-thin films, since the relaxor characteristics may be hindered by substantial leakage currents. In this study, a piezoresponse force microscopy (PFM)-based method for characterizing nanoscale relaxor properties was proposed. Taking ultra-thin Pb(Mg,Nb)O₃-PbTiO₃ (PMN-PT) films as examples, this work compares polarization hysteresis behavior under On-field and Off-field modes of the dual AC resonance tracking (DART) PFM measurements between relaxor PMN-PT and ferroelectric Pb(Zr,Ti)O₃ (PZT) thin films with varying thicknesses. Relaxor characteristics of nanometer-thick PMN-PT films are characterized by modulating amplitude of AC readout to eliminate potential false signals. Furthermore, PFM characterizations of PMN-PT ultra-thin films under different in-plane compressive strains and thicknesses demonstrate that the relaxor characteristics are suppressed and ferroelectric properties are observed at relatively large compressive strains of 3.19%. Additionally, the critical thickness for ferroelectric-relaxor transition is identified. These results verify availability of the proposed PFM-based method for characterizing nanoscale relaxor properties. Therefore, this study not only provides a novel characterization method for exploration of the relaxor in ultra-thin films, but also establishes a foundation for understanding the relaxor polarization behavior in ferroelectric materials, thereby advancing applications of relaxor ferroelectric materials in low-dimensional electronic devices.