The DOI (Digital Object Identifier) is a standard unique identifier that is assigned in the publishing process to articles, some books, and data.
DOI Search Engine Use LibKey Nomad in tandem with Texas State University Libraries to search by DOI and quickly determine if we DO have access, in which case you'll be authenticated, and if we do NOT have access, you can link directly to the Interlibrary Loan (ILL) form to request we send you a copy (for free; no limit on ILL requests).
CrossRef is a resource that allows you to input citation information to see if a DOI (Digital Object Identifier) exists for that resource.
Also search Start Your Research using the "title of the article" and check the record for a DOI.
Google Scholar can also be a place to try search the "title of the article" to see if a citation populates and shows a DOI.
The publisher's webpage for the journal is also a possible source to search to see if they list a DOI for the citation.
It's important to keep in mind that not all citations will have a DOI.