The dead layer near the front surface of aged or damaged NaI detectors produces both an energy dependent diminution in counting efficiency and a characteristic iodine K X-Ray peak which interferes with the registered radiation spectra. In this paper the exploitation of this peak as an internal reference to assess the dead layer thickness and the relative loss of detector quality is proved by means of a comparison between experiments and Monte Carlo simulations. Investigations with different NaI based equipments and photon transport simulations are carried out in order to assess the detector degradation in terms of peak interference, efficiency and resolution.