So is there somewhere that cars on the online registry list whether they are clones or not, or just by searching the threads? - for future reference
One method is to use Newitt's GT/CS recognition guide (hard copy). When he was putting the registry together he included a column on whether he had received a Marti report from the owner or not. So, unless there are typos in the book, all cars marked "M" have a Marti and are therefore genuine. Info on cars not marked "M" (most of them) had no documented Marti provided to Paul, so some of them (most?) are real GT/CSs but some are clones...
The only other way to ensure it's a genuine GT/CS is to order a Marti on any car you're interested in...