I recently noticed that one of my github repos has been forked. I can see the user who forked it under the 'Members' tab, but the graph shown under the 'Network' tab is completely linear, with no indication of the fork shown. Is there any way to determine on which date this repo was forked?
Try using github API (developer.github.com/v3/repos/forks/#list-forks)
there's a "created_at" field, it can be what you are looking for