As already mentioned, if it has a propshaft then it is a 4WD.
As the 4WD mechanism is an automatic one, ie. you cant shift it to 4WD manually it is not easy to tell.
Another way is to check the dashboard lights when you start up, but you have to be quick...
On your dashboard you will see a "warning light" which illuminates and goes off when you start the ignition.
It looks something like (excuse my rubbish graphics)
/----/
!
!
!----!
If this flashes during driving (where 4WD is needed) it is normal (apparantly). If it comes on and stays on, you have a problem with the automatic 4WD system and need to get it checked.
If when you start you car, this light comes on and goes off you HAVE 4WD. If it doesnt come on, you dont....
All in the handbook
Also, it is not a stupid question, it has been asked before (
http://www.chevroletownersclub.co.uk/forum/2x4-or-4x4-how-do-you-know_topic94.html)and been wondered by many of us. But come the winter and the snow, you will know.... just put your foot down, spin the front wheels in the snow and watch (and be ready for) the back ones to kick in... great fun, just make sure you are ready for it