gio info metadata:: path_to/some_file_or_folder
gio set _path_to/some_file_or_folder_ -t stringv metadata::emblems _emblem_name_
e.g. set green checkmark for emblem in Downloads directory
gio set Downloads/somefile -t stringv metadata::emblems emblem-default
gio set Downloads/somefile -t stringv metadata::emblems none
Emblem icons start with string “emblem-*”. Search in /usr/share/icons or right click on some file go to emblems and hover mouse cursor to see the name without the emblem part.
You need to reenter or refresh the Thunar location for the emblem to show up.