The place to look for formal admissions policies is the local LEA website - i.e. the county council or borough. You ought to be able to find this by searching on the relevant name. Policies do vary wildly, even within LEAs - single sex and religious schools might have quite different criteria than mixed comprehensives.
In our borough, six local primaries are designated feeder schools into the local comprehensive that my daughter goes to. Some of these primaries send 99% of their pupils there, others send to a mixture of schools. If you don't go to one of these 6 schools, you have to be the child of one of the teachers, or have a sibling at the school, to get in.
In the next borough, life is more complicated, and several children come to our school in yrs 5 and 6 to guarantee getting into a good comprehensive, rather than take their chance in the selective system operated in their borough (we are about half a mile from the borough boundary ....)
I know we had a chance to look round in Year 5, at an open evening. The other thing we did was to go to the school fair, to see what the place was like relaxed.
Tigermoth, you could always just give the schools a ring and ask them. They all put out prospectuses, and I'm sure would be glad to send you one. Just make sure you don't ring before 10 - the poor secretaries are on non-stop phone answering and dealing with sick calls, late arrivals etc - not the best time for them to explain things to you!
Remember too that community based gossip is very often way out of date. Many people in our local community still tar our (wonderful) primary school with the reputation it had about 15 years ago - they have never been in it, but know all about it. I am sure this works for secondary schools as well.
Ofsted are starting to comment more on the value added element, so more recent reports may be helpful. There website is easy to use, but the reports are very lenghty and turgid. They usually have 20 pages at the beginnning giving an overview of the school, before going into ghastly detail on the teaching of every subject.
On a personal note, I was very worried about my dd starting at a great big school with rough kids - until I talked to a parent from another of the feeder schools, who let slip that she was worried about her kid mixing with the nasty rough kids from our primary! And as I knew those kids, and knew that they were all pretty good kids, I stopped worrying!