in Organize Flashcards, you could go into Edit > Batch, Apply changes to "all cards", then tap "Convert all custom to user dict". After you've done that, you could run a full-text search from the Dictionary (the hollow "E" symbol), which would of course include the user dictionary with all the custom flashcards in it. Dictionary-linked flashcards would be found like this, as well.
Thanks for your answer . I infer from it that in the Search flashcards menu is not possible to search through the Meanings field. I am just looking for a flashcard that I can't find but that method seems too much of a fuss just for that. But I'll keep it in mind in case I really need it.