Hard milling was applied to 100Cr6 steel of 50 HRC hardness using cutting tools with different flank wear. The modified surface layers were investigated by eddy current and incremental permeance measurements at magnetizing frequencies from 0.2 to 10kHz. The signals were studied as a function of a low-frequency magnetic field applied to the sample. Both methods showed growth of magnetically harder peak in the measured curve with tool flank wear and magnetizing frequency increase. New parameters were introduced to evaluate the flank wear and corresponding surface damage in the material. The incremental permeance method showed much higher sensitivity comparing to the eddy current testing.