A gear production plant has, along a gear production path, a station for preparing a number of blanks; a cutting station, where respective teeth are cut on the blanks to obtain a number of semifinished parts; a heat treating station, where the toothed semifinished parts are carburized or nitrided; and a finish station for finish machining the heat treated semifinished parts; the plant also having a check station located along the production path, downstream from the heat treating station or the finish station, and which houses an optical measuring unit for non-destructively determining the effective carburized or nitrided case depth on the teeth.