I think my most obvious sign was the fact that in primary school. When I was asked who I had a crush on during recess one day, I just looked at the nearest boy my age and said it was him. I maintained this "pho-crush" for years, because no one else caught my eye. It was more of a why not just stick with him kind of continuance. Looking back, we were pretty similar people with common interests and friends. Because I was shy though, I never really spoke to him. I think if I had, we could've been good friends.
I think this is an instance where I mistook wanting to become someone's friend and getting to know them better for the idea of a crush. The "crush" spanned from 3rd grade well into high school, before I realized that I was only attracted to his appearance (I guess my "type" kind of grew with him). After I became friends with a girl he had broken up with, I realized that he wasn't actually a person I'd want to date. (The realization that I didn't actually want to be in a romantic relationship with anyone came much later.)