Diagnosing any of these things is tricky, but you are the one with your child more than anyone, so you do all the reading and see what merges best with your instincts about the child you know and understand. Adopted or birth child, I still think parental instincts are number 1.
I do understand it is more difficult without family history to help you connect the dots, but it is what it is.
In addition, once you make a decision and start a protocol, if you see improvement you can feel pretty good about the road. If it makes things worse or doesn't really help (given that kids will still grow and mature without any therapy at all), then you keep searching.
I've told many parents that if they think ASD is a possibility, just try out the strategies we have on this website. If they help, does it really matter if you've been dealing with the right assumption?
_________________
Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).