We individuated a method to distinguish a glass phase from a highly viscous liquid phase in a lattice of vortices, established in type-two superconductors. Our analysis is based on the study of the temperature dependence of numerically obtained 1st and 3rd harmonics curves of the AC magnetic susceptibility, by changing the frequency of the applied AC magnetic field. The harmonics are obtained by integrating the non-linear diffusion equation for the magnetic field, with different voltage-current characteristics, corresponding to the two different phases. This method could be applied to the analysis of experimental curves in order to determine if the detected magnetic response of superconducting samples may be interpreted in terms of vortex glass or Kim-Anderson model. ; Comment: 6 pages, 5 figures, to be published on Physica A