Most DOIs (Digital Object Identifiers) are registered with CrossRef. You can search articles, books, book chapters, and more at CrossRef to see if it has a corresponding DOI. (Not all articles have a DOI assigned.) This can be useful when the source of your citation does not list a DOI (many databases do list it).
You will need to have an account registered under your email address (it's free).
Once you've set up that account, you can use the CrossRef simple text query form, and paste in as complete a citation as possible.