Find the copyright notice in your book (any volume of the Definitive Edition of The Feynman Lectures on Physics). The copyright page is in front, opposite the page titled "About Richard Feynman."
Near the bottom right-hand side of the copyright page there is a line that looks something like this:
1 2 3 4 5 6 7 8 9 10 —CRW— 09 08 07 06
The leftmost number to the left of "CRW" is the number of the printing. (The numbers to the right of "CRW" indicate the years that printing is copyrighted.) For example, the line above indicates a first printing, and the following line indicates a 4th printing:
4 5 6 7 8 9 10 —CRW— 09 08 07 06