> Someone could just have a bunch of forks of other people's work, which they dabble with
is quite easy to spot: those repositories will have "Forked from user/repo" below the repository name (on top of the page, and also in the list of repos of a user). If they committed their dabbling, that is almost certainly on top if you go to the commits screen.
If they committed their dabbling right, and the default branch is still master, it will be in another branch, possibly with a PR attached. It's certainly possible to use a fork master directly, but I think it's a quickly learned lesson to only use a fork master for upstream tracking. (How far behind a tracking branch like that is can be informative, sometimes.)
She said that it was an opportunity to ask a core developer of a lot of open source projects some questions. After looking at his GitHub profile a few minutes before he arrived, it turns out that she believed that all the forks on his account meant that he was an active contributor, when in reality this guy was using forks to save repos he found interesting.
In all fairness to my boss, this guy tried to lead us down that road, since he had contributed a typo fix to a README file a few months ago, and used this as a way to say he was an open-source contributor. Strictly speaking, he is, but there's a big difference in what he's done and what my boss thought he had done.
Given that many developers don't really deal with GitHub all that much, outside of as a link to documentation for hosted code, it wouldn't surprise me if some hiring managers had hired people based on what they've forked, rather than actually checking to see if they've written any code on the account, or whether it's any good.
I never provide or look at anyone else's code before getting to know them for these and many other reasons, but mainly because a repository is in such a vacuum, without context or correlation to what the person really can do.
To paraphrase an old mentor: "Someone with a perfect repository is either faking it or has never taken enough chances."