Please don't send it to Hermes for this purpose. They have clamped down on actual, needed spa requests because so many people are sending bags back for "spa" when what they really want is to check authentication, thus clogging up the repair channels and making it really difficult to get after purchase care for legitimate requests. I have also heard people say that fake bags are confiscated and destroyed, though I do not know if this is true.
There are many reasonably priced services who can determine for you if the bag is authentic. I encourage you to find and use one to address your concerns.
Thanks for your advice. Could you recommend some services sites that can do the authentication for Birkin? Thanks a lot.