Magnetism—the spontaneous alignment of atomic moments in a material—is driven by quantum mechanical exchange interactions that operate over interatomic distances. Some magnetic interactions cause, or are caused by, a twisting of arrangements of atoms. This can lead to the magnetoelectric effect, predicted to play a prominent role in future technology, and to the phenomenon of weak ferromagnetism, governed by the so-called Dzyaloshinskii–Moriya interaction. Here we determine the sign of the latter interaction in iron borate $(FeBO_{3})$ by using synchrotron radiation. We present a novel experimental technique based on the interference between two X-ray scattering processes, where one acts as a reference wave. Our experimental results are validated by state-of-the-art $\it{ab}$ $\it{initio}$ calculations. Together, our experimental and theoretical approaches are expected to open up new possibilities for exploring, modelling and exploiting novel magnetic and magnetoelectric materials.