This paper presents inserexs, an open-source computer program that aims to pre-evaluate the different reflections for resonant elastic X-ray scattering (REXS) diffraction experiments. REXS is an extremely versatile technique that can provide positional and occupational information about the atoms present in a crystal. inserexs was conceived to help REXS experimentalists know beforehand which reflections to choose to determine a parameter of interest. Prior work has already proven this to be useful in the determination of atomic positions in oxide thin films. inserexs allows generalization to any given system and aims to popularize resonant diffraction as an alternative technique to improve the resolution of crystalline structures.