You can normally tell via other clues. For example, the design of the cover, a first edition may say that the book has been nominated for an award, whereas later editions will mention winning the award, an adaption to film, the film's awards etc. Another example might be the material used and colour of the hardback's cover. Even spelling mistakes.
The numbers aren't always reliable though... publishers can reprint multiple times and keep them as the same edition number. So, if you do want to purchase an early edition then do research on the book beforehand.
