Innovative development of a sensor is proposed that uses the principle of Fabry-Perot interferometer. Such a sensor is designed as some kind of contact mirror on which the tested object is placed. This is the same measuring mirror for detecting the high frequency polarization effect of water contained in the material being tested. In that case, the test object is additionally affected by an electrostatic field, which should change the polarization relaxation of water molecules. Since a change in polarization in electrostatic field occurs at the water interface, it is possible to register traces of water in the hidden defects of material. The electrostatic measurement method proposed here can be used to study the internal structure of dielectric materials.