Studies the evolution of the polarization state of an electromagnetic wave propagating in a non-homogeneous plasma with a sheared magnetic field. A differential equation is given describing such evolution in the case of slowly varying plasma parameters. Assuming the model of a cold collisionless plasma one obtains an approximate solution giving the output polarization as a function of the input one and of the plasma parameters along the path. These results are applied to the case of a Tokamak; it is shown that in the some particular cases the output polarization is a relatively simple functional of the poloidal magnetic field. Therefore measurements of polarization can be used to determine the distribution of the poloidal field.