A robust method for the optical characterization of polarimeter optics for a complete, multispectral polarimeter is presented. The polarimeter optics consists of a waveplate, a polarizing filter, and a multispectral detector. The method employs a source of unpolarized light and a rotating polarizing filter and retrieves the retardance and the angle of the waveplate. Three independent measurements at distinct angles of the analyzer are used at a time, and an arbitrary number of distinct measurement triples can be used to ensure the consistency of the results. The elements of the polarimeter optics must not be moved during the measurement, and therefore the method allows one to adjust the setup to defined and potentially optimized values for the retrieved properties. This key element of the method was a crucial feature for the design of the Airborne Multi-Spectral Sunphoto Polarimeter (AMSSP) for the German research aircraft HALO. The application of the method to a single optic of the AMSSP instrument is presented. The variation of the independently retrieved parameters is then used to estimate the maximum uncertainty of the results due to systematic errors.