Experience helps a lot. For example, I will never give money to a Buddhist monk walking the streets asking for donations, er, ever again.
https://www.abc.net.au/news/2015-01-09/victorians-warned-aga...
"Genuine monks do have a practice of having gifts or giving alms as part of their religion but it is meant to be a generous donation of those who wish to give without any solicitation whatsoever."
In a similar way, I'd never work for another startup again. While I'm sure many startups have great intentions, others are out and out scams, and the vast majority will never break even enough to pay you the pot of gold at the end of the rainbow that they promised you. Once burned, twice shy etc.