There are sensors in front of the car. There are also sensors in the B-pillar too. The computer is in the center of the vehicle usually under the center console. When two of the three sensors working in conjunction with each other sense a collision the computer will determine if the air bags need to go off. The sensors in the seats now are for the stage type air bags. It determines your weight and then adjusts the bag to go off in certain stages. So a little old lady doesn't get the same punch in the face as the 300 pound bodybuilder. They also are used to turn that certain airbag on or off. There are 10 airbags on the new Lexus ES just to give you a idea of how many there are in a car now.