The first clue is often in the listing title itself. The brand is written as one word, Montblanc; the split form Mont Blanc — usually combined with an English-language description and a seller based in the Far East — is one of the commonest giveaways of commercially produced fakes.
Invented model names are just as telling. Sellers pair up names that never went together at Montblanc — a StarWalker described as a 75th Anniversary, for instance. The 1999 edition marking 75 years of the Meisterstück has nothing to do with the StarWalker line, and it was never offered with platinum-plated trim. Anyone who knows the correct model names and which trim variants were actually available will weed out a large proportion of listings before even looking at the photographs. Be wary, too, of descriptions that ape an original without clearly calling the pen a replica; there the buyer's mistake is part of the plan.