Q. How do I find the DOI number for an article?

The DOI number is usually on the first page of an article (top or bottom) and sometimes on the last page. Often a database (Academic Search Complete, PsycInfo) will have the DOI in a searchable field and visible from the full citation. CrossRef.org has a free search for DOI numbers: http://www.crossref.org/guestquery/

The APA Style has a quick tutorial:  DOI Tutorial