By like, you know, having a normal conversation. "I'm curious about this project this little repo of yours. Is there some particular reason you chose to use to use / not-use X", for example, where you're asking because you're genuinely curious, as if they actually had something to teach you, perhaps.
If they're legit you'll get a solid answer right away. And if they're not legit... you'll find that out just as fast. But the whole idea is to maintain flow and authenticity. And keep it framed as if you're learning from them, not sitting back and grading everything they say and do on some unseen scale.
And if you're not learning anything? If there's no flow, authenticity? That's when you know it's not going to work out.
Especially people in favor of endless whiteboarding and zombie-like quiz-show sessions.
This kind of question is exactly what I had in mind when I said "pushing to see how you react." I think we agree :)