We propose a method for determining the collimation ratio of a coherent X-ray beam using a planar multilens interferometer. The method is based on the analysis of Talbot images, which are periodic patterns of interference fringes formed by the interferometer at corresponding distances. The high sensitivity of the position and period of the interference fringes to the shape of the X-ray beam wavefront enables determination of its collimation degree and evaluation of the coherence properties of the radiation. The effectiveness of the proposed approach is experimentally demonstrated at the ID15B beamline of the European Synchrotron Radiation Facility (ESRF) synchrotron radiation source. A theoretical study is carried out, and the corresponding results of computer simulation are presented. The experimental data obtained correspond fully to the theoretical estimates.