They can use an array of sensors in the seat and passenger compartment. My car isn't terribly smart so if I leave my bag in the passenger seat it will chime at me incessantly until I shove the bag onto the floor. Cars already use a lot of this kind of information when making decisions around airbag deployment. I was in a frontal accident last year that didn't trigger any airbags because I was firmly belted in and moving at low speed.