All of Sherlock Holmes' deductions about Jabez Wilson are contained in a single short paragraph.
"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."
It is typical of Arthur Conan Doyle to start one of his Sherlock Holmes tales with a display of the great detective deductive powers. The prospective client, Jabez Wilson, is amazed. It is necessary for Doyle to evoke this reaction in this character because he is the only one likely to ask for an explanation. Dr. Watson would not do so at this point because he would not want to intrude into the interview, and also because he would be more inclined to puzzle over the "obvious facts" Holmes itemized and try to figure out for himself how his friend arrived at them.
Jabez Wilson questions Holmes about each of the "obvious facts" he has just deduced, beginning with:
"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."
Holmes explains that he could see that Wilson's right hand is larger than his left, indicating that he had worked hard with it and developed the muscles more. He knew Wilson was a Freemason because he was wearing a Freemason arc-and-compass breastpin. He knew that Wilson had lately been doing a lot of writing because of the shiny condition of the right cuff of his coat and the smooth patch near the elbow on his left sleeve where he had been resting his arm upon a desk. Lastly, he answers Wilson's question about how he knew he had been in China.
"The fish that you have tattooed immediately above your right wrist could only have been done in China. I have made a small study of tattoo marks and have even contributed to the literature of the subject. That trick of staining fishes' scales of a delicate pink is quite peculiar to China."
This sort of thing is typical of Sherlock Holmes stories. The detective comes up with some esoteric facts and then almost invariably claims that he has made a thorough study of the subject and has published an article on it--though why Holmes should want to know about Chinese tattooing is hard to understand.
The only one of his deductions Holmes does not explain is the one about Wilson taking snuff. Wilson asks:
"Well, the snuff, then, and the Freemasonry?"
"I won't insult your intelligence by telling you how I read that . . ."
Holmes does explain his observation of the Freemason breastpin but leaves the snuff-taking for Wilson, Watson, and the reader to puzzle over. No doubt Holmes could see traces of snuff on his visitor's clothing, or on his mustache if he had one. Watson specifies that Wilson's clothes are neither very new nor very clean.
He wore rather baggy grey shepherd's check trousers, a not over-clean black frock-coat, unbuttoned in the front, and a drab waistcoat with a heavy brassy Albert chain, and a square pierced bit of metal dangling down as an ornament. A frayed top-hat and a faded brown overcoat with a wrinkled velvet collar lay upon a chair beside him.
The black frock-coat and the velvet collar of his overcoat, as well as the drab waistcoat, would all be good places to show traces of fallen snuff.
We all have a tendency to make deductions about strangers, which may explain why Holmes' deductions are always so intriguing. Like Watson, we wish we could deduce more than we can.