The airbag is one of those parts of the vehicle that you will likely not notice until something bad happens. Nobody really thinks about the airbag as a critical part, but it could potentially save your life in case of an accident.
The vast majority of airbag-related issues will be indicated by the airbag warning light on your dashboard. When you start up your car, this light will come on, then disappear. If it stays on, or flickers a few times before turning off, you'll need to get your airbag checked out.
There are some cases when the airbag light will not indicate a problem. You need to be especially careful when you buy a used car to check the airbag. If an accident has occurred where the airbag has been detonated, it's possible that it was improperly replaced. Usually the light will come on in this case, but there are instances where the light can be rigged to go off and make it appear as if there is a bag when there really isn't.
Be sure to get a vehicle history report to check into the accident record of the car. This report will make note of whether the airbag was detonated or not.