The printings aren't marked on the notes, so the only way to tell is by serial number. "A Concise Catalog of U.S. Military Payment Certificates" by Carlson Chambliss gives the serial number ranges for the different printings. Each series, and sometimes denominations as well, can vary in the number of printings.. For example, the Series 481 had 4 separate printings of the denominations 5c thru the $1, and 3 printings of the $5 and $10 notes. There is also some discussion as to whether replacement notes are considered separate printings also. The Chambliss book gives guidance on the serial ranges foreach printing. For advanced collectors there is also "World War II Remembered" by Boling and Schwann that specifically speaks to the WWII issues.