In one example a system is for post-processing a three-dimensional (3D) object generated in an additive manufacturing process in which an identifiable agent is applied to a portion of a build material to form a portion of the 3D object. The system comprises an identification unit and a sensor. The identification unit is to cause the identifiable agent to become distinguishable to thereby cause the portion of the 3D object, corresponding to the portion of build material to which the identifiable agent was applied, to be distinguishable from any build material remnant disposed on the 3D object and to which no identifiable agent was applied. The sensor is to distinguish the build material remnant from the portion of the 3D object.