For primary school entry I would leave my child at home. We had to take DD to one school and I could see the difference, I was less able to concentrate as I found I had to make sure DD didn't venture off or decided to take part in the lesson.
DH and I went together to the schools we felt were the ones DD would realistically end up going, we had two very different ones DD would have been able to go to and we wanted both be sure we came to the same conclusion.
Looking out for things: how do the children act when teacher, esp. the head is around. We had everything from getting really quite and not looking up to a Y2 class running towards a head showing her the new display they just finished.
Displays: does it cover a decent amount of variations in ability or just the creme de la creme? Are there displays about the general school life, trips, events held in school?
Is the school offering lunch or after school clubs? Run by themselves or external providers? Do they have sports teams competing on local level between the schools?
Do they offer music tuition and if how does it work? Do they have music groups like a choir or orchestra?
How can you get information from the teacher? Are they open to make appointments with you outside parent evening?
How are they settling reception children in? Some do quite long stretches of short hours for 2-3 weeks, if you work you may need to think about cover.
Also, have a look at the school's website. At ours you would find information about KS 2 residentials, school trips in general, all clubs, the newsletters, a calendar with events and lots of pictures form various visitors to the school.
If the schools do firework displays or have Christmas fairs open to the public it is worth going. We go to two different ones, one is ours, one where good friends have their children and the difference in attitude is interesting. In ours you will find most of the teacher, happily helping and interacting with the children and speaking to parents while at our friend's school it is purely a PTA event and the head is only there because insurance says he must (or a representative from the school).