Epistemologically you can't be completely sure, but when someone's discourse is habitually interwoven with rhetorical gambits and logical fallacies it's not unreasonable to conclude that they're actually a bullshit artist.
Talking to them on the internet/real life over the past ten years
To me, that feels like a broad brush and lazy way to shut down any uncomfortable conversation. If you find it works for you, more power to you