On the other side of things, meeting a lot of markers doesn't always mean a dc will be autistic. My HV was adamant ds1was autistic when he was 3, but he's not (now 6).
He did lots of things on the list - was very speech delayed (pronunciation of an 18 month old at age 3), would completely tune you out, not interested in other children at ALL and difficult to make eye contact with. He wouldn't show me things and seemed happiest left to play alone. He was obsessive about lining things up and making lists and showed what I would describe as OCD tendencies with touching and turning things. When he was 'into' something (at age 3 it was cleaning, playing with brushes, dusters etc) it was all he wanted to do and he could not be dissuaded. I used to take him to play group and had to stop going because he would have screaming, hysterical tantrums when he wasn't allowed to play with the mop or sweeping brush. I couldn't reason with him or get through to him at all when he was like it.
When he was 3.5 (in school part time nursery) we were called in. The teacher had been slowly assessing him and told us he was able to do sums, counting etc at a roughly year 3 level. He also had (has) the strange ability to look at a large group of anything...people, books, smarties...and tell you instantly how many there are. She'd picked it up when they'd had a fire alarm, she was counting the nursery and reception children and dc glanced around the yard and said 'there are 58 of us miss' and was right.
He was assessed for autism again on the teachers suggestion but the result was negative. He's since been listed as gifted, particularly with maths and apparantly the signs of autism/giftedness can be very close.