A test is described, which serves as a self-check of the\noptical part of a lidar system. Several lidar signals using\ndifferent parts of the lidar telescope are compared.\nDifferences in these signals indicate range dependent\ntransmission changes of the optical receiver, resulting\nin signal distortion. A systematic approach including\ncomparison with ray tracing simulations of the optical\nsystem can point out causes of the deviations.