We present a method of beam emission spectroscopy (BES) using an atomic Li beam that is generated by means of laser blow-off for the investigation of electron density fluctuations in the edge and scrape-off layer plasmas of fusion devices. We discuss the operational limits of this technique and compare them to those of other atomic Li beam diagnostics.Furthermore, we apply this method to different Wendelstein 7-AS discharges and study the influence of the line integrated electron density and the direction of the main magnetic field on the fluctuations inside and outside the last closed flux surface, and especially on their movement.The information we obtain on the influence of the main magnetic field on the fluctuations gives some insight into the possible origin of certain features of their spatial structure.