How do you know that a book
is from the Modern Library series?

All editions have the words "The Modern Library" on the book spine, half-title, title page, or dust jacket. All editions after 1925 when the series left Boni-Liveright feature the running torchbearer colophon. There are a huge variety of these colophons. You'll usually find a colophon on the front book cover (especially in editions after 1931), sometimes embossed, and nearly always in several places on the dust jacket.

To see what you should be looking for, check out John Krygier's overview of  bindings and dust jackets from 1917-1939. And of course, get a copy of  The Guide.

Copyright is no indicator that the title is or is not part of the Modern Library series. Most Modern Library series books, being reprints and not original editions, have their copyrights held by somebody else. While other titles have their copyrights held by The Modern Library, not all of these are in the Modern Library series. And sometimes titles that have appeared in the ML series also appear in other series and are made from Moden Library plates (such as Stein's "Three Lives" in the New Classics 50s Series, and Paine's "Selections," France's "Queen Pedauque," Flaubert's "Salammbo," and Ellis' "New Spirit" - all in the National Home Library from the 1930s).

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