  Van der Waals (vdW) magnetic materials are comprised of layers of atomically
thin sheets, making them ideal platforms for studying magnetism at the
two-dimensional (2D) limit. These materials are at the center of a host of
novel types of experiments, however, there are notably few pathways to directly
probe their magnetic structure. We report the magnetic order within a single
crystal of NiPS$_3$ and show it can be accessed with resonant elastic X-ray
diffraction along the edge of the vdW planes in a carefully grown crystal by
detecting structurally forbidden resonant magnetic X-ray scattering. We find
the magnetic order parameter has a critical exponent of $\beta\sim0.36$,
indicating that the magnetism of these vdW crystals is more adequately
characterized by the three-dimensional (3D) Heisenberg universality class. We
verify these findings with first-principle density functional theory,
Monte-Carlo simulations, and density matrix renormalization group calculations.
