I'm assuming it's attached to a mainstream school.
I'd ask how many children are in the unit and if there are others supported informally who increase the numbers.
Are all the children in the unit diagnosed ASD or is it used as a PRU/assessment unit for children with behaviour issues?
Are the children given support to integrate into mainstream or are they taught separately? If they are taught separately is it by TAs or teachers?
How do they communicate with parents? A home school diary which is properly used is essential.
What work do they do with children to support their individual social communication development?
What are their behaviour management strategies.
What would the induction process be and does it respond to the needs of the child?
I would look for staff who bend down to speak to children and really listen to what the children are trying to say.
I'd look for evidence that they make the environment and routines fit the children's needs and wishes rather than expect the children to fit around what they've always done.
I'd want to hear that parents are welcome to pop in and chat anytime.