A geometrical model based on an inverse ray-tracing approach to describe the Ronchi test for a concave spherical mirror is presented. In contrast to the conventional ray-tracing method, which refers to information unavailable in ronchigrams, the proposed model provides an explicit relation between the available information in the ronchigram and the parameters of the setup (radius of the sphere, position of the source, position and orientation of the observation, and grating planes). This allows for extracting the parameters of interest by a simple fitting procedure, as demonstrated by an application. The derived model exhibits new unexplored potential applications of the Ronchi test, establishing it as a very useful, simple, and universal tool for optical evaluation.