Note: Please keep in mind, that all of this does not work if you are not signed in to GitHub.
When searching for issues related to a target I often like to quickly look up their GitHub organization on Google.
So let's say Gratipay says nothing about being open source. A quick Google "Gratipay GitHub" should return Gratipay's org page on GitHub.
Then from there I am going to check what repos actually belong to the org and which are forked. You can do this by selecting the Type: dropdown on the right hand side of the page.