Usually the usage of a Commons file by Wikisource is displayed in the section "File usage on other wikis" of the file description, e. g. for https://commons.wikimedia.org/wiki/File:LA2-NSRW-3-0368.jpg which is used on English Wikisource.
However, for files that are used on German Wikisource this seems not to work, e. g. for https://commons.wikimedia.org/wiki/File:Die_Gartenlaube_%281887%29_894.jpg which is used by https://de.wikisource.org/wiki/Seite:Die_Gartenlaube_%281887%29_894.jpg.
This problem exists for many files, maybe for all files used by German Wikisource.
Originally brought up in https://commons.wikimedia.org/w/index.php?title=Commons:Village_pump&oldid=168443058#File_usage_on_other_wikis_does_not_work_for_dewikisource