[en] Third-generation X-ray synchrotrons like the European Synchrotron Radiation Facility (ESRF) are optimized to produce intense undulator radiation. Insertion devices, such as undulators, ensure the highest possible brilliance--the key parameter for the success of e.g. coherent scattering, which is one of the main techniques employed at ESRF's TROIKA beamline. Nowadays, the constant efforts to reduce the emittance and improve the stability of the electron beam allow using small-gap insertion devices and increase the brilliance. Obviously, it is important to have an experimental technique for evaluating the performance of the undulator source. Here we present a method based on measuring the diffuse scattering from a light amorphous material by a photon counting detector. The measured spectral intensities show a very good agreement with the simulated spectra, demonstrating the high brilliance (above 10²⁰ ph/s/0.1%bw/mrad²/mm²) achieved at modern facilities.