No honest list of the “an original always has this feature” kind can be drawn up. Fakes are revised continually and take over precisely those features by which originals used to be identified with confidence. That applies, for instance, to the “Pix” imprint under the clip of ballpoints, long regarded as a dependable mark of authenticity and now to be found on fakes as well. Such a list would therefore feign a certainty that cannot exist where fakes are concerned.
The reverse approach makes more sense: collect the known marks of fakery and check a pen for deviations in finish, weight, the feel of the materials, engraving, fit and design, ideally in direct comparison with a pen of the same line known beyond doubt to be genuine. Differences in marking are not automatically suspicious: that a ballpoint additionally carries “Pix” while a rollerball does not is perfectly normal, since “Pix” is a traditional Montblanc designation for ballpoints. The execution of the threads likewise varies between lines and years of manufacture and proves nothing in itself, one way or the other. Anyone who needs certainty, before an expensive purchase or with an inherited pen, should have it appraised by the manufacturer; only there can authenticity be established bindingly.