Post Crash Seatbelt Use Investigation
Part of the investigation of any serious motor vehicle collision is an examination of the seatbelts to determine if the person in that seating position was restrained or not. The outcome could determine whether a ticket for failing to wear the restraint was issued or not but I suspect that more commonly the information was used to determine liability. If you were not wearing your seatbelt your award for injury could be reduced by the courts.
The simplest method involved fully extending the belt. Many manufacturers sew a label at the retractor end of the belt that is exposed if the belt is subjected to sufficient force. You may wish to try this if you are considering the purchase of a used vehicle because it could indicate that the vehicle was involved in a significant collision sometime in its life.
Reading the information in the vehicle's "black box" will show the status of the driver's belt switch. While this may mean that the belt was fastened and then just placed behind the driver, damage (or lack of it) to the interior of the vehicle could be used to corroborate or disprove it.
Characteristic damage occurs to the belt and fittings when an occupant is restrained in a crash caused by the tremendous forces involved. Frayed edges, melted plastic smears, D ring impressions and belt fabric impressions are frequently found. The pawl that stops the retractor reel will dent the metal teeth it sits against.
Finally, the investigator can look to medical personnel for evidence. A properly worn seatbelt causes specific injury to the wearer. Thankfully this injury is much less significant than what would occur if the seatbelt was not used.