I would think generally you need to start with defining the priorities, what works, what is to be avoided for your DC needs, so you would have a criteria for comparison and the targeted questions to ask. You also need to know the SEN system and the local landscape reasonably well to navigate.
As Open said, I would look into all documents you could find on the LA "Local offer" website and the school's website concerning SEN, pastoral support, policies. The provisions, the organisation and the ethos could vary quite a lot.
When visiting, take you DC to see how they cope with physical environment, how the pupils and the teachers engage with your DC.
Listen to the Headteacher talk, what do they emphasise? How do they speak about SEN, if at all, how do they talk about helping all children to do the best.
Talk to as many subject teachers as you can, ask how they would support you DC SEN. Do they look blank and tell you to talk to the Senco or do they give multiple examples of children with your DC SEN doing well in their class? How do they respond to you? Are they open, do they engage and want to help, or do they basically believe it is too bad for your DC. Can they talk competently about SEN or are they expecting to never have to deal with them? Ask what the outcomes for SEN children, what systems are in place for extra tuition and progress etc
Do the same with the Senco but in more details. Try to gauge her understanding and awareness of your DC SEN. I came across a Senco that didn't understand anything about ASD and just considered it to be a particularly awkward form of being thick. See how open and responsive she is, how knowledgeable and responsive are other SEN staff.
When you put it all together you would have a picture emerging.