I hope most of you know the tPF has a "FAKE" forum in Chanel Shopping, where fakes we stumble across are posted as a warning. You can learn tons from reading through the posts as to what to look for, etc.. Granted, some of the images are gone because they're links to bags no longer for sale (many were reported and the listings taken down--yay!) But there are enough photos and info to help you become an educated consumer (well, they've helped me and Lord knows I'm blonde and post-menopausal... so if they can help ME...well, you get the idea.)
Chanels aren't the only items subject to knock-offs.
Gucci, Kors,
Coach... Rolex, Breitling... there's a long and sad list. As long as buyers are willing to fund con-artists, con-artists will thrive. IMHO and IMHE (your mileage may vary).
A little education is a good thing when looking to spend money. So is (as had been pointed out) working with reputable authentication services noted on this site.