A welding assembly is provided for welding a fastener to an object such as a vehicle body panel. The assembly includes a weld gun having first and second electrodes movable relative to one another between an open position and a closed position. The first electrode includes a housing assembly. A pin is received in the housing assembly and is movable when in said closed position between a desired nut position, and upside down nut position, and a no nut position. An optical sensor has a terminal end space from the pin for direction light thereon. The terminal end of the sensor received reflected light from the pin. A sensor control system processes the reflected light into a signal corresponding to one of the desired nut, upside down nut, and no nut positions.