The single most important factor is that you like the head. And like as in, you feel you could sit down with him or her and sort out some bothersome issue with regard your child and come away feeling listened to and respected, and have trust in him/her to do the right thing for your child.
Walk past the playground at lunchtime and hometime and see what's going on. Fighting in any form should be a big no-no, or any kind of behaviour that looks as if it might become a fight. Ask how they prevent football-playing boys from taking over the playground, or what they would do if a child seems to be finding it hard to join in with others.
Wrt academic standards, ask by what age the majority of children move onto 'chapter books'. Any time from Reception to Y1 is good, Y2 acceptable, if lots of Y3+ children are still on reading schemes, that is not great. Grab a handful of Y3 exercise books -- by that age they should be able to write reasonably comprehensible prose with intelligible spelling and have a stab at sensible punctuation. Joined-up legible writing at this age is a good sign.
For Reception/Y1, ask how often they change reading books. Once a week is not unusual, twice is better, more often is great (but rare). Ask to see some reading records, and check whether this tallies with what they're telling you. Ask the class teacher and a helper or two, to see if you get the same story. Ask what reading scheme they use. If they use a phonic scheme for teaching reading, ask if the reading books they send home come from the same scheme or not. Lots of schools will feed you a line about using a mixture of methods, which often means they've switched to a phonic scheme in accordance with govt guidelines but haven't invested in the books to back it up.
This reading thing will take over your life for two or three years, so worth finding out the deal before you sign up.