A journal article's unique DOI (digital object identifier) is a type of "permalink" (short for "permanent link") and can be found in the article itself or as part of its citation. If a DOI isn't given, check Crossref (or LibrarySearch, which uses Crossref). Keep in mind older articles might not have a DOI, as the system wasn't developed until the late '80s and many publishers did not adopt it immediately. In these cases, another permalink URL is generally used instead for the citation (for example JSTOR uses their own permalinks).
Journals themselves have ISSNs (international standard serial number) which are similar to ISBNs for books. ISSNs make it easy to distinguish between journals with similar names or to find a journal that has had a title change. ISSNs are particularly useful for expediting interlibrary loan requests. If the ISSN is unknown, check Ulrich's serial directory.
