When an autograph signing is not personally witnessed, it must be authenticated.  At the end of the day, this involves the opinion of an expert who compares the autograph in question against known exemplars that exist either in their head or a computer.  Every person alive cannot sign the same autograph twice.  In a way, an autograph is like a snowflake - no two are alike.  Understanding the nuances of these differences is what makes some authenticators more skilled than others. This skill is usually developed over a long career of selling or collecting autographs.
With nearly four decades of experience, we know autographs. There are very few celebrities whose autograph we have not handled. We have authentic autograph exemplars on file for virtually every celebrity that has had a role in film or television. In short, we know our stuff.