"What would it take to convince you otherwise" is a question I've asked in the past, but I'm less and less convinced of its utility.
If the counterparty knew the answer to that, they would sit down with Google, not engage in an argument. Debate is mainly information sharing, but also to some degree about exploring the answer to that question.
I also like "steelman the other side first" to see where they are and how much they know about "the other side" of an argument. But this only works with people you know and trust to want to go there, not on the internet.
For me, it is really useful: should I talk to this person never again, since they cannot be convinced by any evidence they themselves would find.
Or with close family, should I never bring up this topic again since they perhaps have nothing to gain from changing their opinion, but a lot to lose.
A better phrasing is ‘If you were wrong, how would you know?’. It has the same end state, but positions things as an internal revelation rather than a potential loss of face, so is less likely to trigger a defensive response.
In the same vein, I've been keen to try out "What would the world look like if..." and then show that we do or do not observe related phenomena. It seems like the best way to meet someone on their terms (because they get to write the "rules" of the world) and then you just apply them towards one conclusion or another. But I haven't had enough exposure to really test this out.
For lots of people, logic and facts don’t have much power compared to emotion. Often it seems there’s no argument to be won.