Important indications are a complete documentation and especially matching numbers on the warranty card and on the fountain pen itself. If these papers with correctly assigned edition number are present and pictures and description match known originals, an authentic piece is most likely to be assumed. Especially with rare Limited Editions like the Octavian, such proofs are a central authenticity criterion. In case of doubt, an additional assessment by a specialist should take place.
Related pages on fountainpen.de:
• Patron of Art Octavian (1993): fountainpen.de/patron-octavian-en.htm