I was/am concerned about this as well. Generally speaking, if you buy from a reputable collector (like Xiro or Ace) who has been around on mm or gh, you will be fine.
However, there have been a few fakes sold on mm recently (as fakes), and who knows whether they will fall into less honest hands.
I actually ordered one myself to see if I could find a way to tell if it's fake. Will post any findings.