How do car air bags “know” when to deploy?
Thank you in advance to all who reply 
They are triggered by a sensor that detects impact or very rapid deceleration.
Here is an expanded explanation from How Stuff Works:
Not a bad link Shagnasty but it doesn’t really answer the OP’s question of How do it know?
Air bag systems vary from car make to car make. What I am about to describe is the system used on Volvos, and is very similar to many other car makes.
[Standard disclaimer] This is not how your car works exactly. Do not try to repair these systems yourself. Making a mistake with an airbag system can be both very expensive, and could casue death or serious injury. Do not try this at home I am a trained professional. While I will be happy to answer general questions about the system, I do not want to meet you in the pit cause the bag when off when you were working on the car and you think it is my fault.[/SD]
Inside the car is a device called the crash sensor. The first crash sensors we used had a mercury switch on an inclined ramp that was oriented fore and aft. This mercury switch provides proof that the impact was frontal (or nearly so) In addition there was a piezo electric crystal that generated a voltage in response to the impact. Both of these inputs are hooked to dual processors that work in parallel. Both of them have to agree for a bag to deploy.
In the later cars, the mercury switch was deleted (Mercury is a hazardous waste) and the crash sensors have two piezo crystals that are oriented so that the crash sensor can detect a hit and determine what direction it came from.
In both cases, the hit needs to be frontal or near frontal, and it has to transfer enough energy to the crash sensor to reach the deployment threshold. In other words the hit has to be from the correct direction, hit hard enough, and last long enough that the CS is sure it is in fact an accident. The last long enough is to make sure that if someone hits the crash sensor with a wrench (with the key on) that the bags do not deploy.
In some of our newer models there are sensors located out near the radiator for early warning that an accident has in fact occurred.
I can go on for another 4 hours or so, 
but I will stop here. If you have any questions ask.
Note also that on some of the early models, the systems “guessed” wrong (i.e. were too sensative). I know of several people who had their bags deploy after hitting a pothole at moderate speeds (sure hope that was covered by the warranty). There was talk, and I don’t know if it ever made it into production models of using a pinched tube with a ball bearing in it. During a collision, the g-forces would be enough to pop the bearing out of the pinched part, sending it to the end of the tube where it completed the circuit and caused the bags to deploy.
The ball bearing non electric system was used on some Jaguars, and one year of a Jeep product. It is a vastly inferior system to an all electronic system.
[true story] So there was this teenaged girl that came into the house and told her mom that she was just sitting in the car listening to the radio and the airbags just went off. So mom calls the car company and demands that they fix this defective car. Car gets towed to the dealer, and sure enough there is not a scratch on it. Both air bags and seat belt pretensioners have deployed. Car is then put on the rack. It seems that young sweetums actually had been out driving the car, and drove over a curb stop at speed. Enough speed to drive the entire engine and transmission back about 5". More than enough for the system to determine that there had in fact been an accident. The system was correct, there had been an accident. I have to give it to the young miss, she had the balls to drive the car home and park in the garage and try to guts it out. Too bad it did not work. :smack: The lady got to call her insurance company.[second true story] I had a car towed in with the front wheels pointing at each other. they looked like / \ :eek: It seems that the owner was driving on a street that was not yet paved in a new housing development. The manhole covers were set at what would be finished grade which was about 9" above the then current street level. He thought that hitting one of these with a tire would be a bad thing so he tried to straddle it at 25 MPH. The repairs were over $10,000 and this was back in 1987. There was no body damage done to the car.[/true stories] Point is not all crashes involve body work. With all due respect to your friends, unless I saw the car right after the incident or a very comprehensive report, I am not buying it. It smacks a little too much of a FOAF story for my tastes.
Well, IIRC they all tended to be Ford vehicles, which IME are the favored automotive playgrounds for electrical gremlins. A quick google search pulled up this site which describes a Ford recall to do excessive “unintended deployment” but it doesn’t give any details as to the conditions at the time of the deployments. (Simple corrosion could have caused the problems without a bump being involved.)
Obviously there was a recall, but a recall for moisture intrusion does not support your position that the electronics were too sensitive in early airbag systems and would go off hitting a pothole.