Magnetic fields are a critical foundation for interpreting high-energy astrophysical phenomena across cosmic scales. In this work, we investigate the parametrized post-Einsteinian (ppE) waveform imprints induced by external magnetic fields around Kerr-Bertotti-Robinson (KBR) and Kerr-Bonnor-Melvin (KBM) black holes, to disentangle magnetic signatures from environmental effects in massive black hole binary gravitational wave signals. For a small compact object inspiraling into a massive KBR black hole, the leading-order magnetic correction enters at $-2$ PN, with a spin-induced next-to-leading correction at $-1.5$ PN. For a spinning KBM black hole, the leading magnetic correction appears at $-3$ PN, alongside a spin-induced $-1.5$ PN term. These leading ppE corrections show no intrinsic degeneracy with modified gravity effects, but closely resemble the gravitational pull from power-law matter distributions with indices $\gamma =1$ and $\gamma =0$, respectively. Using a multi-event statistic $F$ to break this degeneracy, we find that robustly distinguishing magnetic effects from matter-induced pull is not uniformly efficient, especially for the KBR case. For the KBM scenario, a clear transition emerges at ${\rho }_0\sim {10}^{-4}kg/m^3$ ($B\sim {10}^{4}T$): above this threshold, the magnetic signature can be reliably separated, while no efficient distinction is achievable below it. Our results highlight that future detections of anomalous $-3$ or $-2$ PN corrections may originate from either magnetic fields or dense matter environments, emphasizing the necessity of combining multi-event statistics and multi-messenger validation for robust gravitational wave astronomy tests.