Britax have car compatibility on their website www.britax.co.uk. Alternatively phone the car seat manufacturer and they should be able to tell you which models are compatible with your car.
The manufacturers can give you a guide as to which of their seats might fit which cars, but the only way to ensure a good fit is to go along to a dealer and try them out in your car. This is a free service at reputable dealers.
There's an explanation of why fitting is preferable by one of the manufacturers themselves on this thread.
If you have a really common car then someone on TB might be able to point you in roughly the right direction.
as i understand it you shouldn't use a car seat in a front seat if it has front airbags unless you can turn off the airbag/deactivate, theres normally a switch or key lock inside the glove box if you can. in my view i always make sure the child seats are in the rear, not just because of the iarbag issue but if the child starts crying drops a toy etc etc its too tempting just to lean over and tend to them BANG youve just hit the car in front.