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Steinbeck means seriousness of the pearl buyers can be shown from the tilt of a hat. The tilt can determine how a man is judged or how much he might receive for his pearl.
It means that rash young men with too much confidence wear their hats cocked back on their heads. There is a difference, even in baseball caps, between the way a mature man wears a cap (shaped bill) and the way a young man wears one (flat bill, etc.). Look around at men in the working world compared to young men who are overly confident... You will notice. Men who wear hats, caps for work purposes can tell the difference between a man and a boy from a distance from nothing more than the way the cap is worn.
Teacher of literature.