Sherlock Holmes' quick eye took in my occupation, and he shook his head with a smile as he noticed my questioning glances. “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.” (The Red-Headed League).
Holmes deduces all of these things from Mr. Wilson's appearance -- namely that his right hand is larger than his left (from manual labor), Holmes smells or sees snuff on Mr. Wilson, Mr. Wilson's breast-pin is of the Freemasonry order (or a violation of that order's rules, as he says), and that Mr. Wilson has been writing because of the shiny spot on his right cuff.
The villain, John Clay, has engaged himself as Wilson's shop assistant. With the help hid conspiratord, they have set up the Red-Headed League to get Mr. Wilson out of his shop for a certain amount of time each day.
“it was perfectly obvious from the first that the only possible object of this rather fantastic business of the advertisement of the League, and the copying of the Encyclopaedia, must be to get this not over-bright pawnbroker out of the way for a number of hours every day. It was a curious way of managing it, but, really, it would be difficult to suggest a better. The method was no doubt suggested to Clay's ingenious mind by the colour of his accomplice's hair. The £4 a week was a lure which must draw him, and what was it to them, who were playing for thousands? They put in the advertisement, one rogue has the temporary office, the other rogue incites the man to apply for it, and together they manage to secure his absence every morning in the week. From the time that I heard of the assistant having come for half wages, it was obvious to me that he had some strong motive for securing the situation.”
During this time Clay and his gang dug from the cellar of Mr. Wilson's shop into the cellar of the nearby bank, in order to steal the great quantity of French gold that had recently been deposited there. The Red-Headed League was only ever a blind and a diversion; the real motive was the undetected construction of a tunnel into the bank in order to rob it.