Can you post your trim tag details? No Van Nuys (VN or LOS on trim tag) cars had X codes, but later Norwood (NOR) cars did. So if the car you're looking at is an early Norwood car, or a Van Nuys car, it won't have an X code.
Later Norwood cars with X tags could only be an RS if X11, X22 or X33.
Plenty of other clues to car possibly being an RS, like stamped backup light holes in lower tailpan, firewall piercings, etc - see the CRG 'Option Identification' section. BUT don't take it for granted that the car is a factory RS just because it may have some of these features....
Option ID link: http://www.camaros.org/options.shtml#id
X tag info link: http://www.camaros.org/numbers.shtml#XnnCodes