Quoting Kayleigh-louise Moran:" was he behind with things like learning to talk, walk, count etc? another sign is struggeling to get ... [snip!] ... does he have a sense for danger?if he does things like runs into the orad without looking, that can also be a sign. good luck x"
All of this.
My son showed no fear of things he should have, specifically heights. He would try to jump off of anything, no matter how high it was. But he expressed fear of things that neuro-typical kids were not afraid of... Water in particular.
He was delayed with speech, had texture aversions, only ate certain foods (PB & J, Cheerios, and spaghetti), did the hand flapping, walking on his tiptoes, looking at things out of the corner of his eye instead of straight on, not pointing when he should have been, did not like to lap read, and had no interest in playing with other kids his age who were not siblings.
As for obsessions, he was obsessed with anything that spun, but specifically wheels on cars or trains. He didn't play with them normally. He would flip them upside down and just spin the wheels or he would line them up.
However, he is extremely intelligent and always has been. He has the memory retention of an elephant. When he was 14 months old, we visited my parents (they lived in another state) and he was playing with a car. It rolled underneath the stand my mom had in the kitchen... We visited again 6 months later and he ran right for that stand, reached under it, and got his toy. He remembered where it was.
He also has an incredible aptitude for all things electronic, as well as planes, trains, and cars.