In this paper, a new kind of approach to reveal the presence of well-defined impurities in isotropic materials is proposed and verified against actual measurements over real samples. The rationale lies in the different polarimetric symmetry properties of inhomogeneous and isotropic materials within well-defined impurities from homogeneous ones. The underpinning physical idea is to inspect the Mueller matrix of the material sample, obtained from an ellipsometric measure: its form, in terms of symmetry, can reveal whether or not if in there are well-defined impurities in the sample.