Always look carefully through these sites. Read their "About Us", "FAQs", "Returns" and "Contact Us" sections closely. Usually just this information will give it away immediately.
Couple of other things to watch out for when you're checking out these sites:
1. Prices that are "too good to be true." I mean, seriously: $875 for a 35cm Birkin?? Gimmee a break!
2. The only authorized online retailers of new Louis Vuitton products are eLuxury and Louis Vuitton's own website. Any other site claiming to sell "new" LV is selling fakes.
3. Sites that charge a restocking fee for returns. This one charges 15%.
4. Sites that use stock photos of the bags. This one includes some stock photos and some that look like the actual bag...but it's something to look out for on other sites. In this site's FAQ section, it specifically states that no additional photos of a bag will be provided. SKETCHY!!!
5. Sites without an address or phone number to contact them.
6. You can look up the website domain registration on whois.com. This website is protected by privacyprotect.org - so the owner is hiding name, address, etc. This can be a red flag too.