By what age do you think they should be reading confidently? (end Y1, ideally, ie. age 6). And what do you do if they don't? How many children is this likely to affect in each year group? What reading scheme do you use? If they've recently changed to a commercial phonics scheme (Jolly Phonics, RML), have they bought reading books that support these schemes? Lots of schools buy the reading scheme, but don't replace their stock of reading books, so kids are learning phonics at school but getting Biff and Chip look-and-say books home. Not helpful.
How often are the books changed in the infants? (Twice a week is normal, more is good, less is a bit slack). Ask the teachers as well as the head to see if replies tally, and ask to see some of the kids' reading records). Who hears the children read at school, and how often? Who decides when children should be moved up a level? If they make v. rapid progress, can they skip levels? These are important questions, cos the whole malarkey around reading books will take over your life for about three years. The whole hearing children read and changing books business is v. time-consuming for schools, so is often palmed off on TAs or volunteer parents, who may have somewhat idiosyncratic ideas about when children should move up, not to mention being completely unqualified to assess children's reading levels My dd1, in Reception, was never, that's never, over the whole year, heard to read by the teacher. I don't think that's usual practice, but it won't be unique either.
Who supervises playtime? How do they prevent football-playing boys from taking over the whole playground? This will happen unless school takes steps to prevent it, eg. zoning the playground, allowing footballs only on certain days. Are playground incidents reported back to class teachers?
How much time in Y2 and Y6 is spent preparing for SATS? Look at some Y6 exercise books and see what actual work (as opposed to practice papers) has been done since Easter. Another good benchmark is to look at some Y3 exercise books and see if the majority of the class are writing reasonably comprehensible prose with sensible punctuation and legible, ideally joined-up, handwriting. Beware of the sleight of hand with which heads and teachers will pass you the exercise book of the class genius. Ask to see a range.
What are the arrangements for children who are beyond the expected level for their year? If they start talking about their G&T groups (gifted and talented) ask for details, ie. how much time per week, what do they do (ask to see it) and who is responsible for it. This is important because primary schools are under pressure to get the maximum number of children to Level 4 by age 11. This is not a demanding target for a bright, well-supported child, and even the higher Level 5 may involve quite a bit of coasting for an able child. There's a tendency to pay lipservice to G&T but it doesn't actually improve a schools statistics, so more able children will often be left.
They'll probably bar you from the premises if you actually ask all these questions 
, but these are the issues that are likely to be dominating your life for the next few years, so you may as well see them coming!