Unpaywall is a browser extension that can be added to Chrome or Firefox to check whether an open access version of an article is available. To download the extension, go to https://unpaywall.org/products/extension.
Once you have the extension, simply browse for the journal article you wish to view on the internet. You will need to find the article as published on the journal website. If an open access version of the article is available, you will see a green tab with an open padlock at the right hand side of the screen. Click on the tab and it will open the open access article in your browser.
The open access articles identified in Unpaywall are ethically sourced from legal deposits. Unpaywall uses open data to power its database, and you can see where that data comes from at https://unpaywall.org/sources.