How do I know if I have the first printing on the Monster Manual or a later printing?

If I go to my local game store to buy a new copy of the Monster Manual (or the Player’s Hand book or …) because my copy is worn out, I want to make sure that I get the latest printing so it will contain all of the latest updates and reversions. But when I get there how do I know which printing of the book is on the shelf?

One way that usually works on D&D books, and most others, is to look for the printer’s key, also known as the number line. You can typically find it on the second or third page just under the ISBN number.

The example shown above on the left is a first edition. The printer’s key is:
9 8 7 6 5 4 3 2 1

Numbers are removed with subsequent printings, so if “1” is seen then the book is the first printing of that edition. If it is the second printing then the “1” is removed, meaning that the lowest number seen will be “2”.

In the example images above the one on the right is the 11th printing.