Interesting question, Tom. Took me a few minutes to find a file that reliably IDs the system, but I think this should work (works on my two different versions of X):
Root level (not user)->System->Library->CoreServices->highlight Finder.app
The version number will be the same as the OS. Another one to check, also in CoreServices, is SystemVersion.plist. You will need to open this in something like BBEdit or TextEdit - it's a text file.