Fluids in contact with attractive surfaces exhibit a wide range of phenomena. Knowledge of these specific behaviors is used to advance technologies ranging from microfluidics to stain defending textiles. For one to describe these phenomena it is necessary to characterize the fluid with its descriptive interfacial tensions and contact angle. Here we propose and validate a new computational technique for determining contact angles and interfacial tensions. Molecular simulations are performed in the grand canonical ensemble with a simulation cell that contains one or two adsorbing surfaces. Transition matrix Monte Carlo methods are used to calculate the grand potential at a given activity, which is compared to the equivalent bulk value to determine the relevant interfacial tension. The contact angle is related to the interfacial tensions through Young's equation. The efficacy of the proposed method is examined by calculating the interfacial properties of a Lennard-Jones fluid in contact with an atomistically-detailed substrate. Good agreement is found between our results and those from a traditional pressure-tensor based technique. Finally, we demonstrate the utility of the proposed method by examining how the interfacial properties of a system evolve with surface molecular roughness.