An example is given at http://www.irishexaminer.com/analysis/fiction-imitates-real-life-in-a-case-of-true-inspiration-172752.html
Bell once remarked to an astonished outpatient: “I know you are a beadle and ring the bells on Sundays at a church in Northumberland somewhere near the Tweed.” “I’m all that,” said the man, “but how do you know? I never told you.” The outpatient left, bewildered. Bell turned to his students: “Did you notice the Northumbrian burr in his speech, too soft for the south of Northumberland? One only finds it near the Tweed. And then his hands. Did you not notice the callosities on them caused by the ropes? Also, this is Saturday, and when I asked him if he could not come back on Monday, he said he must be getting home tonight. Then I knew he had to ring the bells tomorrow. Quite easy, gentleman, if you will only observe and put two and two together.”
Notice that he got information in several ways. The dialect, the callosities and asking about the day.