How did Sherlock Holmes know Jabez Wilson went to China?

Holmes bases his incredible deduction on two crucial pieces of information. First of all, there's the strange tattoo of a fish on Jabez Wilson's right wrist. As Holmes loftily announces, he's made a small study of tattoo marks and has even contributed to the available literature on the subject. Because of this expert knowledge, he's able to deduce that Mr. Wilson received his inking in China, as the scales of his fish tattoo have been stained in a delicate pink color, a design unique to that country. Holmes then refers to the Chinese coin hanging from Mr. Wilson's watch-chain.
Initially, Jabez was rather impressed at Holmes's deductive powers. But now he thinks that there was nothing particularly clever in Holmes arriving at his conclusion in the way that he did. This leads Holmes to lament—half-jokingly—to Watson that perhaps in future he shouldn't explain himself. That way people will continue to be impressed by his astonishing skills of deduction and his reputation as the world's greatest detective will remain intact.