what Bruce is really talkng about is this
except in cases when you absolutely know this is an original still from the original release of the specified movie, it is through the education of dealing with large quantities of this type material over many, many years that you are able to identify what is real and what is not.
people will say "it's an intuitive guess" but what that really means is that "in the opinion of an expert who has vast experience, this item is an original".
this is part of the skill of being an expert, like Bruce, Grey, Sean, myself and hordes of others and it is most definitely why each of us is are among the people you can trust (there are so many others like Ken Schacter, Ron Moore, John Hazelton, Franc in NY etc etc etc)