There are 5 reasons that Sherlock can tell that John is a soldier. Well, anyone who bats an eye can tell but there are things that John does that are not done by most people in everyday life. John is oblivious due, to it practically being cemented in his head during training and his time in Afghanistan. Sherlock finds them fascinating. And cute.
1. The way he sleeps at night.
He had to dig deeper one night as to why John sleeps the way he does. John is used to small beds: cots, just a matress, the ground. He was taught to be "out of the way" and out of sight, exspecually while sleeping. He sleeps straight as an arrow on the right side of his and Sherlock's Queen size bed and he barely takes up his side alone (not that Sherlock is complaining). He covers his abodomin with his left hand, that probably means nothing, hes just comfurtable that way. The only part that looks out of place is his head. It is always turned slightly to the right, never the left. He also sleeps with his right hand tucked under his pillow. Sherlock only realized what this meant when he and John cuddle. At all times, while sleeping, he keeps his right hand in a trigger- locked position. John tucking his hand under his pillow more than likely recreates a buldge that would have been under it due to a small hand gun.
2. The way he holds himself.
John's back is completely straight as he stands. He has more posture than anyone that Sherlock has ever seen. When he walks, he keeps his feet lined in the front, that would be uncomfortable to most unless you were used to it. When he stands he has a habit of crossing his hands, of course alot of people have that habit but John crosses them in the back. When standing his feet are shoulder-width apart. As John walks, he keeps his right hand in his pocket, wieghing his jacket down. Sherlock can make out that is because he is used to the heaviness of a gun on his shoulder and the weight makes him more comfurtable.
3. When he is angry his voice gets deeper.
Sherlock had no idea about this little thing that John does when he is truely upset. When he is at his angriest, his voice gets noticably deeper. It is, as if, droping his voice makes him sound more intimadating. It works. John is frightening when he yells. Him using his "angry voice" as he calls it makes you feel very powerless. Something he picked up to get what he wanted with his men.
4. He eats very fast but not big helpings.
John eats extremely fast. Sometimes Sherlock laughs because it seems like John is trying to eat all of his food before it runs away. In the military, you must eat fast as you are in danger at all times and anything can happen. Its another habit most military men have. You are not served seconds.
5. The way he treats everyday like it could be his last.
This little bit of John happens to be one of Sherlock's favorite things. John is always looking for a good time and is up for anything. There is no stopping him from doing something he wants to do, Sherlock does his best to keep up.
Some of these traits that John possesses scare Sherlock. Most have likely come because of severe trauma, which is something Sherlock refuses to think about. But one thing he does know is that these minor traits make up John Hamish Watson: His army doctor.