Use the GitHub Network Graph to Track Forks
You have no way to see who has checked out your repository using standard git commands such as git clone, but you can see who has forked your repository on GitHub using the Network Graph Visualizer. At the time of this answer, you can access this feature in at least two ways:
- From the "Network" tab just to the right of the "Code" tab on the navigation bar at the top of your repository.
- By clicking on the numbers (if non-zero) in the call-out just to the right of the "Fork" widget on the right-hand side.
For example, here is a partial screenshot of the rbenv network graph:
The "Members" tab at the top of the Network Graph will also show you a different view, listing the names of the people who currently have forks on GitHub. It obviously will not show people who cloned outside of GitHub, or folks who have subsequently deleted their forks.