There's no formula to apply to giving trust, it just happens and sometimes quickly as you say. Women are better at men with the whole intuition thing, but men do it too and that helps a lot. We just feel a certain way and with a little experience here and there, trust happens. We need to apply caution though also, in my opinion.
I agree it's a decent topic to discourse on, broad in scope, and no, I think scammers don't consider it. They probably don't even stop to consider their actions as wrong, greed, ego and hubris tend to blind a person.