MS schools? What are your DS's difficulties? Will he have a statement, need 1:1?
How will you support my DS with his language/social skills/toiletting/behavioural/educational/medical issues? Etc etc What training would they be prepared to offer to relevant staff?
The toiletting one can be surprisingly challenging for a MS school, it'll give you a good feel for their willingness to do the right thing. (If it's an issue of course)
Language, does he have SALT? Would school work with SALT following recommendations? Does anyone (TA, teacher) have experience of taking small groups in SALT activities?
Social skills, does the school use circle of friends, does it have involvement with ASD outreach etc. Could it support your DS at lunch and break to get involved in some structured games.
Behavioural, how would the school handle meltdowns, tantrums? Would it appreciate that NT methods of sanctions may not work and may be counter productive. Have they used ABC analysis methods to help recognise triggers (antecedents) and apply helpful consequences rather than ones which are self defeating? Is the school secure? Could your DS escape from the playground, classrooms and leave the premises?
Educational? Really depends on his difficulties, how long is a piece of string?
Medical? Experience of a care plan, similar to toiletting, staff prepared to be trained in use of equipment, giving medicine?
There's loads more, I'm sure.