Instructions
Look for the Montblanc trademark "Pix" on the pen. Many counterfeit items very closely resemble an authentic item, but if it does not have the "Pix" then it is not real.
Examine the nib for the words "iridium tip." The only Montblanc pen with the words "iridium tip" is the Starwalker pen, which has those words on the nib and is silver. All other Montblanc pens do not have those words on the nib. (The nib is the tip of the pen that distributes ink when in contact with a writing surface.)
Look for an inlaid six-point star on the top of the cap and examine the barrel of the pen. It should be made of natural resin and should shine nicely. Examine the underside of the nib, as the black section will be in line with the ink aperture. There should not be any deviation. If it is not aligned, the item is likely a fake.
Inquire how many pens the seller has if not purchasing from an authorized Montblanc retailer. If the seller has more than four pens, the pens are likely not authentic because the price of purchasing Montblanc pens in bulk would be prohibitively expensive.
Be cautious when purchasing Montblanc pens online. Ask for lots of close-up pictures of the pen at various angles. Also, ask for written assurance by the seller that the Montblanc pen is authentic. Then take the pen to an authorized retailer or antiques dealer to verify authenticity. If it turns out to be a replica, you can then get your credit card company or the selling website involved.