Capturing the structure and properties of water at the interface with a solid surface presents a unique challenge for both Kohn-Sham and classical density-functional theory due to a complex interplay between chemical binding, electrostatic interactions, steric effects, and dispersion. X-ray reflectivity measurements probe the electron density of aqueous interfaces with high precision, but rely on model-dependent fitting to obtain the corresponding structural model. We present a validation protocol which enables calculation of interfacial X-ray structure factors from theory for direct comparison to experimental measurements. We apply this protocol to benchmark joint DFT, classical molecular dynamics, and first principles molecular dynamics simulations of the Al2O3/water interface, probing the effect of pressure, temperature, and finite size upon the predicted structural model. We explore the relative strengths and weaknesses of each class of theory, gaining insights into the bonding and structural properties of interfacial water which will aid future development of more accurate electronic and classical density-functionals.