It was actually others who pointed out I might be a lesbian when I was 17. This was because in talking about my past, I mentioned many boys I was with, but it was more like we were friends who were playing a game of let's pretend (though I might tolerate some sexual behavior, I quickly got bored with it), and no boy ever broke my heart, nor was I ever jealous of a girl trying to steal my boyfriend, which I actually found funny.
In contrast, I was having sex with my best friend, and it hurt my feelings when she said it was "just practice" and "didn't count."
Though I don't recall crushing over anyone on screen (at least not until I was 17), I do recall some possible subtle clues. For example, as a child (not even hitting puberty yet) I read
The Cat Who Wished to Be a Man over and over, and as the character made his love clear for a woman, I was right there with him. The book was practically told from his point of view (not first person, but I think he was in every scene) so I just assumed it was my identifying with the protagonist was just natural, but in retrospect I realized I didn't identify as strongly with female characters doing the same with men (though I might sympathize with them and hope it works out, my imagination and identification just wasn't as into it), ironically making it one of the few "romances" I liked (but it's filed under fantasy, not romance).