We use a non-intrusive technique based on a polarization scheme to probe the periodic alignment of the CO2 and N2 molecules induced by a strong laser pulse linearly polarized. A weak probe field experiences the birefringence resulting from the alignment of the molecules. By comparing the probe depolarization signal with the numerical simulation of the time-dependent Schrödinger equation, it is possible to quantify the alignment of the molecular sample. The modelling takes into account the spatial profile of the pump intensity. It allows from the alignment signal to determine the onset of saturation due to ionization, despite the intrinsic saturation of the alignment arising for about the same laser intensity.