Locate the Soldier's Honorable Separation Report. That document was issued to the Soldier if the Soldier survived the war. Separation documents provided proof of service for later services/needs.
If you are a military veteran or next of kin, you may submit a request to the National Personnel Records Center in St. Louis, MO for information from your, or your relative's, military personnel records.
NARA maintains a website explaining the National Personnel Records Center policy on releasing limited information from the Official Military Personnel File (OMPF) to the general public.
Military personnel records are accessioned into the National Archives, and become archival, 62 years after the service member's separation from the military. This is a rolling date; for example 2018, minus 62 years is 1956.