How do you know if that person is sending the signal to you that they have interest in you?

I remember a guy in high school that was so clueless that a girl was hinting at him to take her to senior prom for months that she finally told him outright a week before, he actually told her that he should be the one tonask her, she never spoke to him for the rest of the year and shunned him at the graduation dance. Three years later he came out as queer as a three dollar bill and became a headliner at the local Drag Queen cabaret.