lol I managed to stay in denial until I was 18... In high school, we had "initiation" where any girls who wanted to join our group had to kiss me (not really, but we joked about it, because I made out with most of the girls who joined our group), but I always wrote it off as something we just did because the guys liked it.
Finally when I got out of my parents' house, I realized that I REALLY enjoyed those makeout sessions and would love to explore it more. 2 years later I accepted that I could potentially fall in love with women, too, when I developed very strong feelings for a close friend. So... first clue was probably kissing a bunch of girls repeatedly - even though I didn't think about it at the time.