As a fan and owner of several genuine signed copies of Meyer's work I feel that it is necessary to make a statement about the increasing number of blatant forgeries that have been posted on eBay in direct correlation to the rise in Meyer's popularity. While I am not an expert I believe that I have established somewhat of an eye in this regard after watching listings of Meyer's signed work for many months and purchasing several copies from trustworhy sources. I have listed a few key points below in an attempt to provide whatever help that I can.

1. Stephenie Meyer's signature has evolved over a relatively short period of time.
2. The author's earliest signatures appear to have been complete and comparatively compact. Some later signatures also appear relatively elaborate but these seem uncommon, such as books that might have been signed outside of mass signing events.
3. One distictive difference among Meyer's signed editions appears to have occured with regard to first printings of Eclipse. Most of these books bear the simplest form of Meyer's signature, almost only two initials, and were predominately done in black ball point pen, as opposed to the thicker, darker sharpie. This is not to say that early printings of Eclipse could not have been signed at more recent events.
4. The most easily recognizable consistancy in Meyer's signature has always been the shape and size of the top of her "S" in Stephenie. If this looks comparatively unusual, there is a good chance that it is a fake.
5. Many forgeries appear to be an attempt at a hybrid of early Eclipse signatures (because they are easily reproduced) and Breaking Dawn signatures (comparatively large and signed in sharpie).
6. Familiarize yourself with the dates and locations of various signing tours and compare signatures from events of a similar period.
7. Please remember that photographs can be reproduced and unusually explicit claims of authenticity (such as challanges to utilize authenticity tests) seem to go hand-in-hand with forgeries.
8. Many of the most apparent forgeries, though not exclusively, seem to offer unrelated "extras" such as buttons, cards, and posters which can be purchased almost anywhere to entice unkowing buyers. Some legitimate listings also utilize this tactic but, perhaps, view this tactic as a red flag.
9. PLEASE DO NOT BE SOLD ON "PROOF". Additional proof is obviously nice, but remember that you are not purchasing photographs and flyers. Many of the most apparently legitimate signatures provide no additional proof at all. Always compare the signature with as many others as can be located.
10. Ask the seller detailed questions. Some copies have been purchased second hand and the details may be lacking but good sellers will provide swift responses and will be accommodatingly forthcoming.
11. Ask sellers if the photograph is of the actual book to be purchased. I know of at least two sellers that possess multiple signed copies of Meyer's work and have reused the same photographs to depict honestly genuine signatures on different books. I have asked these sellers myself, and they have been honest with me about why they have reused the same photograph. However, if a purchase is made and the item recieved looks like a sloppy forgery while the listed picture appeared legitimate, ask the seller why and, if their response is unsatisfactory, immediately request a full refund.
12. Always make sure that the seller has a hassle free return policy.

Stephenie Meyer's signature has changed over time but it is fundamentaly uniform. Look at as many listings as you can, ask sellers questions, and utilize your best judgment. Remember that you are not purchasing "proof", you are trying to purchase a genuine signature. These books are not cheap, so do your due diligence as a buyer and keep thieves from profiting as a result of a lack of better judgment.

I hope that this helps, even if only a few buyers.