In this paper, we apply the multi-frequency eddy current testing for nondestructive testing of products made of non-ferromagnetic materials. Such method is extremely complex to process and analyze obtained results of control. However, it is the most promising method for measuring and computing complexes handling a large number of experimental data. The paper provides an experimental validation of various properties of aluminum alloy products affecting the output parameters of the eddy current sensor in a wide range of frequencies of an exciting electromagnetic field. Digital processing techniques with instantaneous values approximation by the signal source model function are applied. Parameters of the model function are derived by least square method. This approach allows us to eliminate noises and significantly improves the measurement accuracy. Obtained results demonstrate that proposed technique of measurement results processing produces fairly reliable hodograph plots and appears to be promising for multivariable control of nonmagnetic materials.