If you don't see a DOI on the article itself (usually on the first page of a PDF version, or sometimes in a header/footer or citation) or on the webpage of the journal, you can try searching on crossref.org to see if one has been assigned.
Select the 'Search metadata' tab. Searching the article title, in quotation marks, should return the article information and show a DOI if one has been assigned.
If you don't see a DOI in any of these places, it's possible that the article doesn't have one. While most major publishers now assign DOIs to published works, it is not required that all publications have a DOI. Older articles, including those that were originally published only in print, may not have DOIs.
For guidance about how to reference an article that doesn't have a DOI, check out this FAQ: How do I reference a journal article in APA Style (with no DOI)?