The visualization of magnetic features can be of forensic relevance in the field of tape authentication. The Research Institute for Material Science and Technology in Zelenograd, Russia, has developed crystals with magneto-optical properties which permit convenient, precise and non-destructive visualization of magnetic information. These crystals change their optical properties according to external magnetic fields. The magneto-optical effects may be used for developing images of the magnetic structure of any analogue or digital recording. In this paper, we first provide some theoretical background about several magneto-optical effects. Subsequently, two experiments on visualization effects that are based on different physical principles (Kerr Effect, Faraday Effect) are described. Selected forensically relevant features detected on analogue recordings are shown. Special emphasis is given to visual features that may be used for authentication purposes.