[AnimalMuppet]

100%? You don't.

But when you see people confidently stating unreasonable things in short posts, it's a common tip-off for a bot/shill/zealot/troll. When they assume a starting point that is two standard deviations out of most peoples' Overton Window, and they don't defend it, just kind of "Of course X is true", that's someone who is blasting propaganda, not having a reasonable discussion. They just want to say "X is true" enough times that people start to believe it.

As I said, that's not 100%. For any individual instance, you can look at their posting history. But for a comment section, when you start to see such posts taking over more and more of the discussion, you can say that the discussion has been taken over with very close to 100% certainty, even if you can't be certain about any one post.