For some articles in databases, you can find the DOI by clicking on the article's title and looking at the information about it:
If you can't find the DOI on the article's information page, you can look it up with Crossref's DOI lookup tool.
If neither of these methods works, be aware that not all articles have DOIs. Some citation styles require DOIs if available, but if one is not available, you don't need to include it.
