"Pshaw, my dear fellow, what do the public, the great unobservant public, who could hardly tell a weaver by his tooth or a compositor by his left thumb, care about the finer shades of analysis and deduction!"
It's saying that there is something very obvious about a weaver's tooth and a compositor's thumb that gives away their occupation
A weaver has an extra tooth on account of the way the thread is used. A compositor has a worn down thumb due to the clicking on buttons for compositing applications.
If you just look at the footnote on the Google Book Search page, you will see the following explanation:
"Weavers often cut thread with their teeth, thus wearing them, while compositors or typesetters often developed calusses on the left thumb, which they used to hold the type in place on the compositor's stick."
(As far as this anonymous contribution: "A weaver has an extra