They say x, ask for where they heard about x. They say from y news organization, ask the source for y. If they say z, if z is actually an actual document or otherwise, ask them where in z does y claim x is true.
If z is just another news organization, keep going until they provide (or don't provide) an actual document. If they can't procure an actual document, send it yourself. If z is just another video cIip, ask them for the whole video. Provide them the entire video if they can't.
Do not try to antagonize. Just simply ask the questions as if you were trying to understand their point of view, to learn something we might not have heard.
Ask a liberal what opinion you are allowed to have that runs counter to their own. So instead of stating what you think or believe, ask THEM to give YOU an opinion that they would allow you to argue. And the liberal will quickly realize that there are ZERO opinions you are allowed to publicly hold that run counter to their own. I mean really, name one. Pro-life? Nope, that’s hate. Pro-border? Nope, that’s racist. Pro-Republican? Nope, that makes you a bigot. And I think once you’ve trapped the liberal into admitting that NO ONE is allowed to disagree with them on ANYTHING, it’s immediately obvious who the close-minded ones are.