PubMed
When searching PubMed, look for icons that indicate free full-text is available for the article.
Citations to articles available via PubMed Central, free full-text archive, will include this icon:
Many records in PubMed will include an icon from the publisher. Sometimes this icon will indicate free, full-text.
If the record includes an icon, always click on the icon to see if full-text is available. In this example, the full-text article is available at the publisher's site, even though there is no indication in PubMed. Click on the icon or the DOI (digital object identifier to access the article at the publisher's site: https://doi.org/10.3928/19404921-20180223-01 The article has an "open access" icon, 
Unpaywall
Install a free extension, https://unpaywall.org/products/extension, in your Chrome or Firefox browser. If free, full-text is available, the "green" unlocked padlock icon will appear. This tool may not be 100% accurate in identifying articles that are freely available, but it is another visual cue that an article is available.
When you have Unpaywall installed, if you find an article at the publisher's site and full-text is available, you will also see the icon. For example, https://doi.org/10.2337/cd17-0078 is freely available.
Google Scholar
When searching for articles, Google Scholar (http://scholar.google.com/) can be used as a supplement. Unlike PubMed or other databases, there is no quality control in terms of the content that is retrieved in a search. Results may include citations to journal articles, books/book chapters, conference papers, dissertations, or scholarly work that has been deposited in institutional repositories. Evaluate search results carefully.
A useful feature of Google Scholar is there may be full-text links to articles that may not be reflected in database searches in PubMed or other databases. Look for HTML or PDF links to the right of the citation.
