1. Look for an index page whose pagelist tag contains a language code (like <pagelist 36to533=it 566to576=it /> on https://en.wikisource.org/wiki/Index:The_Oxford_book_of_Italian_verse.djvu )
2. Open the corresponding index page in the other language (in this case https://it.wikisource.org/wiki/Indice:The_Oxford_book_of_Italian_verse.djvu )

I. Observed: all the "foreign" pages in (1) are linked in blue, including pages which don't exist yet in the "source" language subdomain (in this case, it's a series of "it it it it it..." blue links).
II. Observed: some blue links (e.g. https://en.wikisource.org/wiki/Page:The_Oxford_book_of_Italian_verse.djvu/200 ) show the image and the header «This page does not need to be proofread. Its text comes from it.wikisource.org. Pagina:The Oxford book of Italian verse.djvu/200» (the name of the "foreign" page is shown, but no text is transcluded).
III. Expected: the index page should list missing foreign pages in red and return a 404 for them if accessed, like the source domain.