WPML String Translation only seems to search in the original language (English) and not in existing translations.
So to find which english string corresponds to an already translated text, you can do this:
SELECT * FROM wp_icl_string_translations, wp_icl_strings WHERE wp_icl_string_translations.value="Translated text in any language!" AND wp_icl_strings.id = wp_icl_string_translations.string_id;