A system includes a seatbelt retractor and a webbing payable from the seatbelt retractor. A marker is fixed to the webbing. A proximity sensor is fixed relative to the seatbelt retractor. The proximity sensor is designed to detect the marker when the marker is within sensing range of the proximity sensor. The marker is positioned along the webbing to be out of sensing range of the proximity sensor when the webbing is engaged with a 6-year-old anthropomorphic test device and to be in sensing range of the proximity sensor when the webbing is engaged with a child seat.