You definitely can't trust people's self-evaluation either way. Lots of really skilled people are self-critical. "Reading people" of course helps but it is nowhere near enough.
But you don't have to become an expert. If I hire a plumber or carpenter, I cannot do what they can. But with enough work I can understand which plumbers or carpenters will do a good job.
I wonder if the people who become fantastically successful simply have the ability to evaluate expertise, without having any of it. i.e. by "reading" people, in some sense.
This relies on the "expert"'s own evaluation of their expertise, which mightn't be accurate, because of Dunning–Kruger, or conversely, self-doubt. So, one would need to "read" those too.