When reviewing an article in the database, you will often find the DOI at the bottom of the record. It will not be present for all articles. If you do not see if, please look for a full-text version of the article.
UseCrossref, a free site, to look up DOIs. The free DOI lookup searches for DOIs using information such as article title, authors, and publisher information. Look up on DOI Website: CrossRef.org