Observations with ground based telescopes are affected by differential atmospheric dispersion due to the wavelength-dependent index of refraction of the atmosphere. The usage of an Atmospheric Dispersion Corrector (ADC) is fundamental to compensate this effect. The design of an ADC is based on atmospheric models that, up to the best of our knowledge, were never tested on-sky. We present a new method that will allow us to measure the sky dispersion in the optical range, based on the determination of the position of the PSF centroid at each wavelength of each spectral order. The method is validated using cross-dispersed spectroscopic data acquired with the slit spectrograph UVES. Using the method we will be able to compare different atmospheric models, with an accuracy of ~18 milli-arcseconds (mas).