Determine if the book has any special interest to any particular group of people. It may be a distinctive treatment of a historical event, or it could be the first edition of a new series that has the potential to "take off" in time. Use your intuition and your own sense of what you like.
Find out how many editions were included in that first run. The value of any book is directly related to the number published in the first run. The process works on basic free market principles: when the supply falls short of demand, the value of the book will go up.
Look for printing errors on your copy. Unlike stamps and other collectibles, where mistakes in image quality can make the value go up, book collectors usually don't want books containing errors. If you find duplicate pages, upside down text or other mistakes, return the book to your bookseller for a new copy.
Check the condition of the book. If the book is in poor condition, it will lose value no matter what the other factors indicate. Books are rated "poor" if they contain ripped or loose pages, tears, creases or any damage to the cover. A book in "good" condition may have obvious flaws, but makes a good reading copy. You would rank a book's condition as "very good" if it has one or more small defects, maybe a tear or two, but nothing serious. The more valuable collectibles are ranked as "fine" (no real defects, but not in perfect condition) and "very fine" (perfect in every way).
Look beyond the simple ratings. When purchasing a first edition book that has been reviewed for its condition, note any plus or minus signs alongside the various ratings. These signs offer a more nuanced approach to the grading and value. "Fine" with a plus is better than just "fine." Think of it as a B+ grade. Also, note that at times there are double ratings for a particular book. For example, if you see VG/F next to a book, that means that the book itself is Very Good, while the dust jacket is Fine.