It's funny looking back there were tons on signs but I can't remember when it really hit me that I was different. Probably around 13 I started not to fit in well and started feeling really shy and awkward. Around 10 I put on a lot of weight (despite being pretty active) that never left which certainly helped to exacerbate everything.
As to early signs, I watched all the superheroes and science fiction of the day. People talk about all the superhero movies these days but I had live action Wonder Woman, Spider-Man, Superman, Hulk, Shazam and Isis, Six Million Dollar Man (and his Woman). Had Star Trek, Star Wars, Flash Gordon, Land of the Lost and Battle of the Planets and Star Blazers. Not to mention King Kong, Dracula, Godzilla and more. I can remember drooling over 4K computers in the back of Radio Shack catalogs and my second-grade teacher inviting me to play some artillery game on a TRS-80. And all this before I was 8.