How do you tell the different between people in public who are unstable but keeping it to themselves, as opposed to those who are potentially threatening?

I was on the subway the other day and there was a man who was obviously....not right...yelling things like "who wants to play casino?" or talking about how his mom cooked chicken and dumplings.

I could tell that I had nothing to worry about with his man. He was in his own world, but he was ok in that world. I chalk it up to a lifetime of experience in NYC.

It got me to thinking...I don't even know how to articulate what the difference is between him and someone who may be truly dangerous, because I'm sure there were people on that train car who he freaked out.

And often I read accounts on this sub about "crazy people" on the train, when the majority of the time it's people like this. Acting weird, but not a threat.

How would you describe what to look for when assessing a threat, as opposed to just being aware of someone acting strange?