How do I tell if it is a First Edition?

The publisher may actually state the words ‘first edition’ or ‘first printing’ on the copyright page. Another common method of identification is the number line – that’s a line of numbers on the copyright page. Usually, if a one is present in the line then it’s a first edition. This style has been used since World War II.

The line sequence could ascend or descend or even have no discernable order depending on the publisher. All of these sequences below are first editions.

1 2 3 4 5 6 7 8 9 10
10 9 8 7 6 5 4 3 2 1
1 3 5 7 9 10 8 6 4 2
(All first editions)

Sometimes the number line is also accompanied by the words ‘first edition’, but be careful because some publishers leave on the words ‘first edition’ even when the book is in its third printing and that fact is reflected in the three in this number line.

First edition
3 4 5 6 7 8 9 10
(Third printing)
This number line below identifies a second printing printed in 1975.
75 76 77 78 79 10 9 8 7 6 5 4 3 2

If you find that the date on the copyright page matches the date on the title page, then it is probably a first edition. Most 19th century publishers placed the date of publication on the title page but that practice faded out after 1900 and the date became appearing on the copyright page.
Some publishers make no statement at all about first editions but booksellers learn to identify firsts by other methods – for instance a particular piece of copy on the dust jacket or a mistake in the book’s text itself that is corrected in later editions.

(From ABE Books)