I have a four year old who is diagnosed as autistic and a 21 month toddler who I have starting the process for.
My older son I was more certain, there was almost no eye contact. He never pointed (still doesn't), he waved at one point but stopped, no shared attention, no interest in other children, hated changes in routine, no imaginary play, no speaking. He was so different from other children when we went to Stay and Play.
My little one is more engaged, has more eye contact. It lead me to think he wasn't at first. He even plays with his brother (my older son once used to him really took to having a baby around). However he has no speech, he doesn't point, has no shared attention, is obsessed by things that spin, he has no imaginary play and he is weird about things being done the right way.
It is hard to say in a list, there is just a something about them and I can see it. The official people take some convincing though, with my eldest I was told I was paranoid and now I am being told it is just a speech and language delay.
If it is any help to you my eldest attended a special needs nursery and has just started a special needs school. They are doing so well with him and he is so happy. He gets upset on days he can't go to school. He has made so much progress and achieved so much.