One reliable and frequently overlooked test is to compare the item with what an edition actually comprised. Many fakes come unstuck because they show versions that never existed: a Writers Edition F. Scott Fitzgerald in blue, a rollerball of the Greta Garbo Edition, which by collectors' accounts was never made at all, let alone in steel, or a ballpoint belonging to an edition that appeared only as a fountain pen. Check before buying which types of writing instrument, which colours and which materials belong to a given edition, and pieces like these give themselves away at once, with no detailed knowledge of the workmanship required.
Harder to judge are fakes that stick to models which genuinely exist. The worst affected are comparatively recent, much sought-after ranges such as the Bohème, the StarWalker and the Greta Garbo Edition; here nothing helps but close examination of the clip, the star, the nib engraving, the lettering on the ring and the finish, and, in case of doubt, the verdict of a specialist dealer. Auction listings from unknown sellers therefore always call for extra caution.