Just a note on terminology: a first edition can exist in multiple printings. If no changes are made to the text, then the 2nd, 3rd, ...38th printings are technically all first editions. A second edition occurs when changes are made to the original text, as with Women. What you want are first printing copies. These are indicated by the color title pages. As mjp states, that about 99.9% of a reliable indicator, but I recall something like a second printing that someone here had or saw with a color title page.