Instructions
Look at the nib, which is the tip of the pen where the ink flows. The underside should have a black section that is aligned with the ink aperture. If it is not aligned precisely, the pen is probably not authentic.
Examine the pen closely for the company trademark "Pix." If the trademark is not on the pen, it is not an authentic Montblanc.
Hold the pen up and look at the barrel, which should have a shine and reflect light well. If the pen is a Montblanc Starwalker, the nib should have "iridium tip" inscribed. Note that this is only true of Starwalker pens.
Look at the top of the pen cap for an inlaid six-point star, which is a feature of authentic Montblanc pens. If the pen does not have a six-point star or the star has more or less than six points, the pen is a fake.
Be wary of online sellers, as many Montblanc pens offered online are not authentic. Ask the seller to provide detailed, up-close pictures of the pen, including the nib, the top of the cap and for the Montblanc trademark. If the seller does not comply, the pen is likely not authentic. In addition, require that the seller offer a refund policy and/or satisfaction guarantee.