Reflection high-energy electron diffraction is a widely used tool to study the growth dynamics of thin films, in situ and operando. Here it is applied after deposition, revealing that the morphological state of the surface is connected to the amount of material underneath. We focus on the growth of LaAlO3 thin films on a SrTiO3 substrate by pulsed laser deposition, utilizing a geometrically anisotropic plasma plume. After deposition, oscillations in the intensity and width of the specular spot as a function of position are observed along the short axis of the plasma plume. Each complete oscillation represents a unit-cell difference in film thickness, as confirmed by local x-ray reflectivity measurements. We apply this approach to demonstrate the critical thickness for conductivity at the LaAlO3/SrTiO3 interface on a single sample. Our results introduce a straightforward method to engineer and study thickness-dependent properties of epitaxial thin films on a highly detailed level.