  We present a method, based on noncollinear second harmonic generation, to
evaluate the non-zero elements of the nonlinear optical susceptibility. At a
fixed incidence angle, the generated signal is investigated by varying the
polarization state of both fundamental beams. The resulting polarization charts
allows to verify if Kleinman symmetry rules can be applied to a given material
or to retrieve the absolute value of the nonlinear optical tensor terms, from a
reference measurement. Experimental measurements obtained from Gallium Nitride
layers are reported. The proposed method does not require an angular scan thus
is useful when the generated signal is strongly affected by sample rotation
