The function of a radial shaft seal is based on interactions between the sealing edge and the shaft counterface. To better understand these mechanisms, a look inside the sealing contact was taken. The procedure and the testing facility are presented in this article. In addition, a research methodology was developed to systematically analyze the fluid flow and the distortion of the sealing surface of shaft seals. The results of the test procedure are documented in an evaluation matrix that allows the comparison of sealing systems with different worn sealing edges. For example, the tests show an influence of the rotation direction of the shaft during running-in on the formation of leakage channels or oil collecting chambers within the sealing contact.