We present novel high-speed and mode-resolved polarization measurements with PM and non-PM Ytterbium-doped fiber amplifiers up to their TMI thresholds. The implemented full-Stokes polarimetry technique is based on the simultaneous detection of four parallel channels on a high-speed camera. It enables spatially and mode-resolved polarization analysis with sub-ms temporal resolution, allowing to monitor the polarization of individual modes during TMI. We believe that this high-speed mode-resolved polarization measurement technique is highly interesting for the characterization of PM fiber lasers and could help to explore and analyze new TMI mitigation strategies.