Many things to look out for, these are just a few:
How big is the school, how many children in each year - small probably (tho' not definately) means all teachers know all the pupils and your child is more likely to be treated as an individual. Large means more likely to get in!
What sort of settling in do they do for new children e.g. my dd will go (with Playgroup) for a few afternoons in the summer term. When she starts school in September the Reception class has its own toilets, and don't go to main school assembly until summer term. They also have their own playground so are quite 'closeted' for the first year.
How well equipped is the library, do all children have access to it, and when (if can only go for an hour a week not mcuh good, but if they can get a book every lunchtime and afterschool that's obviously much better.
Do they have computers in one room only or one in each classroom. Are the teachers familiar with how to use them! Do the children get to use them regularly.
How big are the classrooms? Is there lots of childrens work on the walls? Are they bright and airy, or poorly lit and stuffy?
Are the teachers enthusiastic? Is the Head supportive of the teachers and vice versa? How welcome are parents? Can parents get involved in the School, through the PTA or whatever?
How often do they have assemblies? How much religious worship is there might be an important question to ask.
Do they offer extra-curricular activities? What range is there and when? (Reception, year 1 and year 2 children do not normally join such clubs but will be useful to know for later).
What are the outdoor facilities like? Small playground or large, sports field or not? What are the toilets like and how many are there? what are school lunches like?
When you visit are their children wandering around aimlessly, are they all sat in silence concentrating hard (personally I think somewhere in between is better - a hum of activity just seems more child-friendly than silence!)
Also you can look at league tables!
School will be your child's main source of a social life so look at the children going there? Are they the sort of children you want your child to mix with?
Does the school have an anti-bullying policy (it should, even if bullying isn't a problem). How you they approach discipline?
Above all does the school feel right for your child? Some of the above may not be important, and some will be more important than for others. If you have another child who will also go to the same school you may want to think about what's best for them both (using your crystal ball obviously!).
There are probably lots more things you could find out, I'm sure others will add!
Good luck!
PS - I think this is my longest posting to date!