Teaching autistic children to learn and do work independently is difficult. How can we find someone who thinks outside of the box (to learn) and helps those children be motivated to learn when there is not necessarily intrinsic learning value (set up reward or motivation system).
I think the first thing to do is make a list of the qualities that you're looking for. Inquire with the child's school about possible therapists that could be of assistance. I agree that you will get a good sense of the person when you interview them. The key is to see if they match your list and have a good interaction with your child. When you need a therapist to work with an autistic child the key is to find a match with the child.