Well i had a few obvious clues....the blue interior is usually a dead giveaway, also mine still had the steel SSP rims on it and a full size steel rim spare in the trunk. Aside from that, my car doesn't have a roll bar or spotlight so until i found the buck tags i wasn't really sure. You need to look for yours and see if you can find them and go from there. If not you can decode the VIN and that'll tell you some as well but the buck tags are the real icing on the cake.
This page tells you just about everything you can ask for
I've never seen any that went as far as going into the trunk either. The only ones i've ever seen were a simple loop going up the sides behind the seats and around the roof to add support in case of a roll over. They weren't a roll cage just a single bar.