Start going to school events that are open to the public - there'll be lots of christmas fairs coming up.
This is a very good way of looking round school buildings and seeing the children and parents off duty. If you can strike up a conversation with a few of the parents, you should get some home truths about the school.
You could also try contacting local kids clubs, like brownies, cubs, sports, drama clubs, etc. Make sure they do not have an intake from one school only. Say you are new to the area and ask if someone can advise you on the local schools.
Whatever you do, though, do not take one person's view as gospel especially if their child has only gone to the one school. You might meet a parent who has different children at different schools or has moved children from one school to another. If so, get talking!
As you say, you will look round schools so you can to form your own opinion as well.
IME, a good headteacher makes a lot of difference, so try and find out what sort of reputation the head has.
Going on my experience of SE london schools, if in doubt still, choose a smaller school over a larger one.
Will add more if I can think of anything!