I do this all the time. I'm alway picking out people and just wondering if they might be trans. I don't know why I do it, especially since I'm not one to quickly judge in other ways (race, religion, etc.). Like for instance, I'm taking a biology class right now titled "Human Sexuality." It doesn't just cover male and female anatomy and physiology, it covers behavior - social behavior regarding sexuality of all sorts and masturbation, as well as trans issues. I always find myself wondering if someone takes this class because it is something they relate to. I know some of my queer friends take it just because they want to learn what other people think about them and their behavior. Although most (if not all) of my friends who are also taking it are gays/lesbians. There is this one person who I think is a trans guy, besides me of course. I haven't talked to him/her so I don't know their name or anything and it is a large class. I also do this in public constantly. but mostly I pick out women with masculine faces (or so I think).
But mainly I think it is my biased opinion when looking at people, and I want to meet other people irl who are like me. I only know one trans guy other than myself irl, and they are much further along in transition than I am.