I don't think you have to do much to prepare.. maybe if there are some things you don't know about how you acted when you were a kid, try and find that out.
I got diagnosed when I was younger so the tests and things might be different, but I remember a lot of weird questions... like on one written test I was asked if superman was real. It's not a trick question, so don't say yes if you get that, LOL. (I wasn't sure..) And just a lot of simple questions about, I dunno, what I liked to do and what I was scared of and stuff.
I was also made to do a lot of puzzles, and play some games, and draw pictures of people and houses and stuff. And point to dots on a screen as quickly as I could. It's all pretty basic stuff that just shows the guy how you think. But I really don't know if you'll do any of those things specifically.
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"You gotta keep making decisions, even if they're wrong decisions, you know. If you don't make decisions, you're stuffed."
- Joe Simpson