Differential atmospheric dispersion, due to the wavelength-dependent index of refraction of the atmosphere, will affect ground based observations. In order to correct this effect, the usage of an Atmospheric Dispersion Corrector (ADC) is fundamental. Insufficient, or wrong correction of the atmospheric dispersion, will produce a spectrally elongated shape instead of a circular white one for the observed target. The commissioning tests of ADCs with on-sky observations are not an easy task. In fact, the residual dispersion is expected to be of a few tens of milliarcsec, where the object for a seeing limited telescope is almost 1 arcsec. A procedure was developed, based on ellipse fitting of several cuts from the guiding camera images. The characterization of the data allows the validation of the ADCs alignment by determining the dispersion direction and by minimizing the ellipticity. The procedure was tested on ESPRESSO using the guiding camera images, and was able to correct a small error of a few degrees in the ADC prism alignment that was not possible to infer by other means.