I don’t think the author had a chance to interview each person they observed to see if their worried were right. That does not mean that they could not validate their observations.
You look at patterns and note them. Over years you will see similar patterns, both in people you only observe once, as well as people you get to know better. The ones you get to know better are the ones where you validate your theories. I’m not explaining it very well, but it works. It’s kind of like a sort of sparse sampling or a very long-term Monte Carlo simulation in n-dimensions (that’s an allusion and not a strict explanation)
Anyone who has had to learn social skills/cues as an adult, with analytical faculties in place of a more intuitive understanding, can attest to these seemingly universal patterns. People are ultimately more predictable than they believe themselves to be, and there are clear signs of a rich (evolutionary?) history of habit formation over many generations that in a real sense define "the human condition". One should consider themselves privileged not to have been forced to spend their cognitive efforts on understanding these patterns rather than merely be socialized into them.
You look at patterns and note them. Over years you will see similar patterns, both in people you only observe once, as well as people you get to know better. The ones you get to know better are the ones where you validate your theories. I’m not explaining it very well, but it works. It’s kind of like a sort of sparse sampling or a very long-term Monte Carlo simulation in n-dimensions (that’s an allusion and not a strict explanation)