If a character holds the 'Planegazer' title, it means they have completed the Opticording Sphere Quest, making it one way to check. Note, however, that The Supreme Cube is obtained before the quest ends; therefore, a character might not have the title but could still possess the cube.
You can also use the search field to look for specific items:
Or in the Item Summary section of a character that is up for auction:
