When Holmes questions Mr. Wilson he learns that his assistant showed him the advertisement, he is not very young, he’s smart, and Wilson can’t pay him as much as he’s worth.
When Mr. Wilson shows Holmes the advertisement, he realizes that something odd is going on. He immediately notices that Mr. Wilson only keeps one assistant, and he is smart and gets paid little. This catches Holmes’ attention and arouses his suspicions.
His name is Vincent Spaulding, and he's not such a youth, either. It's hard to say his age. I should not wish a smarter assistant, Mr. Holmes; and I know very well that he could better himself and earn twice what I am able to give him….(enotes etext pdf. p. 6)
Holmes continues, and learns that the assistant spends a lot of time in the cellar and takes a lot of pictures, which he supposedly develops there. Mr. Wilson also notes that since he lives at his business he rarely leaves the building.
After hearing the story of the league, Holmes also learned that Mr. Wilson’s assistant came in response to an advertisement, dozens of others applied, and he was chosen because he “was handy and would come cheap.” Holmes asks if it was half wages, and Mr. Wilson says it was.
Holmes then asks what he looks like and gets a description.
Small, stout-built, very quick in his ways, no hair on his face, though he's not short of thirty. Has a white splash of acid upon his forehead.” (p. 12)
He then asks if his ears have been pierced, and is told they were. That settles it. Holmes says nothing, but he is fairly convinced he knows who the man is—the infamous criminal John Clay.