How do you distinguish a mirror from not a mirror on GitHub?

I often have my git configured to push to multiple upstreams, this means that basically all of your mirrors can be primaries.

This is a really good part about GitHub. Every copy is effectively a mirror, too, and it's cryptographically verified as well, so, you don't have to worry about the mirror going rogue without anyone noticing.

I use GitLab locally and push only to that. GitLab itself is configured to mirror outbound to the public side of things.

In a collaborative scenario, doing it that way makes sure everything is always properly synchronized. Some individual's lacking config can't break things.