It's ridiculously hard to get tested as an adult. You'd almost certainly need to pay out of pocket, in the neighborhood of two grand. I started the process when I lived on the other side of my state; I met with a psychologist who specializes in autism, and then a neuropsychologist. It was kind of funny, actually; she'd ask me a question during the interview, I'd get halfway through my response, and kind of falter, and she'd fill in the rest for me, and I'd practically scream, "YES!! No one else understands!!" LOL. She had me scheduled for the official testing process, but when we learned my insurance wouldn't cover it, I canceled. I did eventually take a free adult screening through an autism center. You had to score in the neighborhood of 60 for them to say, "Yeeeaaahh... it's highly likely you're on the spectrum... we'd be happy to recommend further resources for you." I doubled the score. I'd honestly love to get an official diagnosis. Self-diagnosis is highly respected within the community, but I'd like to have that vindication for any doubters.
Yep, my daughter was around 18 months when I first thought she might be autistic. This was about a year prior to me recognizing it in myself. Her symptoms were more classic (i.e. she stims more obviously than I do; she struggled tremendously with toilet training; she has more social anxiety, etc.) I "know" how to act socially, but it's exactly that... an act. I find it draining. My symptoms are more classic "Asperger's" (strange interests; memorization of probably 150+ birthdays; extreme introversion due to lack of commonality with most folks (one reason I like this forum; it's so clearly full of bright ladies!); spatial difficulties, among others.)
You're so correct about teachers and social workers. I won't argue that there are dedicated, intelligent teachers, but they can be few and far between; and even more so if we're talking about a bright kiddo. Your gal is most likely brighter than 90% of the authority figures who'd be in charge of her at a public school. She's very fortunate that you brought her home. And I have zero doubts she'd fly through the GED.