Any suggestion that the mechanism is a fake is quickly ruled out on examining the mechanism at the National Archaeological Museum in Athens. From the state of the remaining fragments, it is very clear that the mechanism has been under the sea for a very long time. Further evidence that it is not a fake is literally emerging from inside the mechanism itself. Much more evidence of the purpose and function of the mechanism has been revealed by the recently acquired CT data. This includes detailed inscriptions which would have been almost impossible to fake.
Read more in this article by Alexander Jones: http://www.antikythera-mechanism.gr/node/774 (Appendix on the authenticity of the Antikythera Mechanism).