The experimental documentation of the properties of an optical system represents a particular challenge. Besides the measurement of focal quality and field distortions, telecentric systems have to yield a parallel beam propagation direction. In this paper we propose a method to test, document and optimize the telecentricity of a laser scanning system by scanning two crossed polka dot beam splitters at once. By separating both beam splitters in Fourier space we were able to detect tilting angles below 2 · 10^-3 rad for four different laser wavelengths within the same optical system. By this we determined the optimum system parameters for our scanning laser optical tomography (SLOT) setup.