Many times, especially with newer articles, you can find the DOI listed in the article itself. You might have to look around for it, but usually it is at the top. Example:
You can also find it listed on the detailed record display page. This is the page you see when you click on the title of an article from your search results. It shows more details about the article like source info, subjects, the abstract or summary, etc., the DOI is listed near the bottom. Sometimes it is just the DOI number, but sometimes it is listed as a link. The DOI number is the last part of that link.
You can also find it when you click on the Cite link in the toolbar (on right hand side or top). Scroll down to the APA reference format, and you can copy the citation and paste into your paper. The DOI is listed as the last part of the citation.
If you still can't find it, you could try the free DOI lookup website: