A friend of mine as just had a head on crash in a ford focus MK 2.5 09 reg speed about 40 mph some 18 year old lad 3 time over the limit, all ok just a bit of back and neck pain. but the thing is the air bags did not go off.

there was no light on to say that the air bags had a fault.

so i would like to know how you would go about checking they will work when needed

The drivers seat is fitted with some sensors that detect seat position such as how close to the steering wheel the driver is. If the driver is too close the air bag wont go off as it will do more damage than it helps. it also depends where the car was hit. Checking the car for fault codes might so something although it may not be enough to trigger a dash light

I summersaulted and rolled my peugeot 307 3 times. With all around impacts including the roof...none of the drivers airbags went off just the passenger curtain one.

Apparently the sensors need to a right smack square on before they fire the bags. Maybe didn't hit the sensors hard or square enough.

Sorry to hear the news and im glad your mate is ok. I hope the police throw the book at the drunk driver.

+1 on John's comments. Unless you hit them right, they wont deploy.