A high precision rotating ellipsometer was used to perform a systematic study of the pretransitional orientational order induced by the free surface of n-alkylcyanobiphenyl liquid crystals (it = 5-12) in the isotropic phase. All the compounds of this series show a complete wetting behaviour at the free surface. The temperature dependence of the surface order parameter was determined via calculations based on the Landau-de Gennes theory and the results show an even-odd behaviour. Moreover, we used a neural network recognition algorithm to reconstruct the order parameter profile S(z) near the surface. The results of this model-independent analysis are compared with the model based calculations.