How to look at secondary schools for a child with SEND

When your child has special educational needs (SEN) it feels like there is an even greater weight to getting the right school for them. Many children with SEND can attend regular mainstream primary and secondary schools and have their needs met well there. Both our boys have an ASD diagnosis but only one went to a mainstream primary school.  When it came to his secondary transfer what would I share about looking at secondary schools for him?