Early in "The Red-Headed League," Sherlock Holmes surprises his friend Dr. Watson, as well as his new client Jabez Wilson, with the following deductions.
“Beyond the obvious facts that he has at some time done manual labour, that he takes snuff, that he is a Freemason, that he has been in China, and that he has done a considerable amount of writing lately, I can deduce nothing else.”
Wilson, a simple-minded man, is especially surprised. He asks:
“How, in the name of good-fortune, did you know all that, Mr. Holmes?” he asked. “How did you know, for example, that I did manual labour. It's as true as gospel, for I began as a ship's carpenter.”
“Your hands, my dear sir. Your right hand is quite a size larger than your left. You have worked with it, and the muscles are more developed.”
These were still the days of wooden sailing vessels, and ship's carpenters were essential members of the crews. They not only had to keep the hull in repair but were always working on the masts, spars, railings, tackle, and everything else. The fact that Wilson had developed such a large right hand indicates that he pursued that line of work for years. No doubt that was how he got to China. In other words, he was away from England and out of touch with his homeland for many years.
Sir Arthur Conan Doyle not only wanted to display his hero Sherlock Holmes' powers of observation and deduction, but he evidently wanted to use the same brilliant deductions to explain why it would have been possible for Jabez Wilson not to have heard anything about the "Red-Headed League," although his assistant tells him it was the talk of the town at the time it was founded. Wilson is a character who is made to order for the part he has to play in the story. He tells Holmes:
"I am a very stay-at-home man, and as my business came to me instead of my having to go to it, I was often weeks on end without putting my foot over the door-mat. In that way I didn't know much of what was going on outside, and I was always glad of a bit of news.”
He is the perfect dupe to suit the purposes of John Clay. Wilson was out of the country for years, and he rarely leaves his shop with attached living quarters now that he is back in London. He is very tight with his money, so he doesn't subscribe to a newspaper or buy one off a street-vendor. His difficulty in finding the ad in the newspaper he has brought to Baker Street shows that he is not much of a reader anyway. Even though he has brilliant red hair, he has never heard of the Red-Headed League; but he is anxious to hear about it when his assistant brings it up, because he likes money and is in need of more since his pawn-broker business has been falling off.
When Watson first sees Wilson he describes him as "...a very stout, florid-faced elderly gentleman with fiery red hair." Such a man would be extremely unlikely to venture down his steep, rickety cellar steps to see what his assistant was doing down there. Holmes also points out that Wilson uses snuff. This is a finely powdered tobacco, and a heavy user would no doubt have breathing problems along with the high blood pressure indicated by his "florid face."