To identify a Kodachrome slide in your relatives’ old film boxes, use this reference page (credit: Andy Romano):
There have been isolated mentions from people who find slides in old boxes and aren’t sure whether they have something in Kodachrome. Kodak were generally very good about marking their film lab processing when it was Kodachrome. Third parties also developed Kodachrome in the U.S. after 1954. Identifying those as Kodachrome might take more effort.