The nib is, and remains, the single most important point of comparison. If the engraving, the shape and the platinum plating all match a documented original of the same model, that is a strong argument for a genuine pen. The second place to look is under the clip, where the word Pix must be stamped. On the Rubber Line fountain pen the rear end of the barrel is also finished in steel colour; on fakes this end is frequently black all the way through. The fit of the rubber sleeve and the crispness of every engraving repay a close look as well.
A new StarWalker comes with a service guide and an engraving voucher. Neither proves authenticity, since papers of that kind are copied just as readily as the pen itself; conversely, a missing till receipt is no mark against a pen, because Montblanc will repair one for a fee without any proof of purchase. Only the manufacturer can settle the question for good: in a boutique or through customer service a pen can be assessed reliably from its nib, its trim and its ident number.