Abstract: The usage condition of a vehicle seat lap/shoulder belt is determined by detecting the proximity of the belt to at least one of the seat bight and back cushion. A passive component is embedded or otherwise attached to at least one of the lap and shoulder portions of the belt, and at least one active component is disposed behind the trim fabric of the seat to detect the proximity of the passive component to the seat. The proximity measurements are used in conjunction with other sensor data such as belt tension and/or state to reliably detect the presence of a rear facing infant seat and to discriminate between proper and improper lap/shoulder belt usage.