During regular gameplay in Sun/Moon, Ultra Sun/Ultra Moon, and Scarlet/Violet, although the in game models for the shiny forms of Minior are identical, the menu sprites of Pokémon don't show shininess, but do show a Pokémon's respective form. Therefore, you can simply check the menu sprite Minior has when in your party or in your box, which should show its color. It's likely they just checked for which form those Minior had for any of those games respectively to otherwise know they got the shinies of all seven forms.