The most telling feature is the depth of the facet pattern. On the original the grooves are cut noticeably deeper and produce a crisply drawn, even relief that breaks the light sharply. Copies, by contrast, look flat and soft; the pattern appears rolled on rather than cut. The second reliable indicator is the section: on copies the shape, the length and the transition to the barrel differ visibly from the original, and the front part often looks too stubby or too little tapered.
To assess a pen on offer, compare it directly with the manufacturer's official product images or with a piece known to be genuine, using photographs taken from a comparable angle. On this model, discrepancies in the depth and regularity of the guilloche and at the joins between section, barrel and cap are the most dependable clues, because a metal surface can be judged comparatively well even from photographs. If doubts remain, only submitting the pen to the Montblanc service will settle the question of authenticity for good.