Most books will say if they are second, third, fourth etc editions.
Plus.
Copyright page of a book, located on pages at the beginning of the book before the story starts, there will be a row of numbers at the bottom of the page.
The lowest number in the sequence tells you the edition number. The row of numbers in a first edition looks like this:
10 9 8 7 6 5 4 3 2 1
Or : 1 3 5 7 9 10 8 6 4 2
Example, 20 19 18 17 then this is the 17th printing.
If you need advice on a valuable book or to find if it is valuable you could try Brian Hartles at Goldsworth Books http://www.goldsworthbooks.com/
Ria