“You appeared to be surprised when I told you, on our first meeting, that you had come from Afghanistan.”
“You were told, no doubt.”
“Nothing of the sort. I knew you came from Afghanistan. From long habit the train of thoughts ran so swiftly through my mind that I arrived at the conclusion without being conscious of intermediate steps. There were such steps, however. The train of reasoning ran, ‘Here is a gentleman of a medical type, but with the air of a military man. Clearly an army doctor, then. He has just come from the tropics, for his face is dark, and that is not the natural tint of his skin, for his wrists are fair. He has undergone hardship and sickness, as his haggard face says clearly. His left arm has been injured. He holds it in a stiff and unnatural manner. Where in the tropics could an English army doctor have seen much hardship and got his arm wounded? Clearly in Afghanistan.’ The whole train of thought did not occupy a second. I then remarked that you came from Afghanistan, and you were astonished.”

Arthur Conan Doyle (1859–1930) British writer and physician

Story (1886-04), “A Study in Scarlet,” Part 1, ch. 2, Beeton’s Christmas Annual, Vol. 28 (1887-11-21)
    (Source)

Holmes and Watson. Published in novel form 1888-07.

In the Sherlock TV episode 01x01 "A Study in Pink" (w. Steven Moffat) (2010-07-25), this explanation is reworked:

HOLMES: When I met you for the first time yesterday, I said "Afghanistan or Iraq?" You looked surprised.
WATSON: How did you know?
HOLMES: I didn't know, I saw. Your haircut, the way you hold yourself, says military. But your conversation as you entered the room said trained at Bart's, so army doctor. Obvious. Your face is tanned, but no tan above the wrists: you've been abroad but not sunbathing. The limp's really bad when you walk, but you don't ask for a chair when you stand, like you've forgotten about it, so it's at least partly psychosomatic. That says the original circumstances of the injury were probably traumatic: wounded in action, then. Wounded in action, suntan: Afghanistan or Iraq.