I’m left-handed, and the only time I have had callouses on my left-hand fingertips was when I played guitar.
(Mundane) case in point: Earlier this year I was watching the July Fourth fireworks at Liberty State Park (on the Jersey side of New York/New Jersey, guess what famous landmark is nearby) and was chatting with one of the law enforcement/park service types there. I noticed he had something very similar to this anchor tattooed on his arm, which was about about as much a giveaway as having this, and so in conversation confirmed he had been a Chief (now retired).
My fiance is a chef and he has very characteristic callousing on his right (knife-wielding) hand. He often has straight line burns on his arms from hot pans, oven racks, etc.
I tried this myself some time ago and so came to realize the importance of Watson. As a doctor and therefore a man of science, Doyle always had Watson size up visitors first. He did so, usually commenting on the color and style of their clothing, their possible rank, dimensions, etc. A lot of blather which a reader might think was complete. Then Holmes would move in and describe all the things that Watson missed.
If it weren’t for Watson’s description, Holmes could easily recognize an American cowboy. “Good God Holmes! How could you tell?”
“Elementary Watson. I could tell by his pointed boots, his broad hat and pair of six shooters.”
For modern clues try The Mentalist on TV. He comes up with some good ones now and then.
My mom (67 years old) calls those straight line oven-rack burns “newly-wed burns” but recently were were talking and we have decided that they are more appropriately “newly cooking for one’s self” burns. Mine were aquired in my first apartment, my brother got his working in a kitchen at age 15, both of these happened about 20 years ago, long before either of us married.
Two successes I’ve had in real life. The first time we visited our new neighbours’ house, we were having wine and snacks. I asked which of them was left-handed. They thought it was remarkable, but the cutting board was on the right side of the counter, the baguette was on the right side of the cutting board and the knife was on the left side. Pretty easy, really.
My wife went to a new hairdresser, and she’d had a long chat with him. Based solely on the fact his name was Cyrus and that he was from somewhere in India, I was able to tell her that he was from Bombay. It was an inspired guess from the fact that Cyrus is a common Parsi name and the largest population of Parsis is in Bombay.
And of course, if I’d been wrong, I wouldn’t be quoting these in this thread, now, would I? Sadly, Sir Arthur did not leave us many instances of Holmes being completely out to lunch on one of his deductions. It’s too bad; it would have made Holmes a little more human.
Holmes a human? Blasphemy! 
Well, for what it’s worth, if you looked closely at my right hand, you might be able to deduce that I bought a new Mac recently. Three guesses as to how—it’ll be fun!
Any chemist who’s got that scar is very, very sloppy. I don’t know any chemists/ChemE who have it.
Nava, ChemE, ex-Orgo TA
People from different countries in Europe use their cutlery in different ways. For example, in Britain I often see people scooping up small objects (think peas) while the knife is used to prevent the objects from escaping: in Spain that would be done with the fork staying still and using a piece of bread to push the objects onto the fork. This second method requires bread with a solid crust, like the breads that are traditional through much of southern Europe but not in the UK or the US.
You’re right. Holmes is a Vulcan.
The Mac mouse has an embossed logo of some sort which has left an imprint on your hand?
Possible spam reported.
CMC fnord!
No Holmes-style ability needed for that deduction.