1. A method of determining the state of a restraint system provided within a vehicle, the method comprising: incorporating a plurality of marker components into a belt restraint of a restraint system located within the vehicle; receiving sensor data from a sensor disposed within the vehicle in a position in which the sensor can observe each of the plurality of marker components, wherein the sensor data comprises information as to respective spatial positions of the plurality of marker components and as to whether each of the plurality of marker components has been detected by the sensor; determining, with a processor, a state of the restraint system based on the sensor data, wherein the state of the constraint system comprises an extent to which the belt restraint is deployed for an occupant of the vehicle that is using the restraint system, based on the respective spatial positions of the plurality of marker components; and determining, with the processor, a size of the occupant of the vehicle that is using the restraint system, based on the respective spatial positions of the plurality of marker components and based on whether each of the respective plurality of marker components has been detected by the sensors.