The only way to know for sure (aside from the original window sticker) is through Marti Auto Works:
http://www.californiaspecial.com/resources.htm#g
For as little as $17 you can know for sure.
Now, there are certain clues you can look for on the car. Although anyone can fake these things, it's tough to fake them all.
1. All GT/CS cars begin with a VIN of "8R01," meaning a '68 San Jose-produced coupe.
2. Any of the special parts identified under "Features" and "Parts List" on this site. Navigate to these pages using the list of links on the left of the page. The more original (i.e. non-reproduction) features that it has the greater the likelihood that it's authentic. One of the best ways is the part number found on the taillamp harness's purple tag.
3. Post some digital pics for us! We loooove to play "spot the flaw" on pictures of cars that are supposed to be GT/CS's.
Hope this helps! If you have any specific questions about the authenticity of a part, please let us know.
-Winston