A first and often decisive step is to check against what the edition in question actually comprised. The rollerball for the 100 Years of Montblanc edition came in black only; versions with green elements do not exist in the original. The Patron of Arts edition Humboldt, in turn, was issued only as a fountain pen, so a rollerball or ballpoint in that decoration cannot be genuine from the outset.
On the instrument itself it is worth looking under the clip. Montblanc rollerballs and ballpoints carry the markings Germany and Pix there, raised as a three-dimensional relief, not milled and not lasered in. Flat engraved or lasered lettering in that place is a strong indication of a fake. To that come the usual signs of poor manufacture: clips that sit badly or feel too soft, cap rings without crisp definition, uneven lacquer, ill-fitting threads and a body that feels light and cheap overall. If in doubt, take sharp close-ups of clip, cap ring and emblem and compare them with documented originals, rather than relying on the general impression given by small sales photographs.