A blog explaining how the serial number were generated:
https://www.amyjohnsoncrow.com/how-to-decode-a-wwii-army-serial-num...
The serial number search page of the U.S. National Archives and Records Administration (NARA):
https://aad.archives.gov/aad/display-partial-records.jsp?s=3360&...=#
(if the soldier's serial number is only 7-digits include a 0 at the start to make an 8-digit number for searching)