Your local authority will produce a booklet about secondary transfer which is given to all children in the primary schools in that authority. It lists all the open days and admission criteria.
If you border another local authority you will have to get a copy of their booklet - they are usually available in local libraries. Most schools are happy for you to look around during the day but you will need to call them individually to arrange it.
I think that playground tittle-tattle is actually useful here as other parents with older children will have already gone through the process or have children at the local high schools.
As a secondary teacher, I would say that different schools work for different children. Some children will really like exam-driven hothouses, some might like a school where the results aren't so great but the pastoral care and general ambience are good. I think it's a case of going with your gut.
You need to go with an open mind and don't discount schools with poor results, as there intake may be starting from a low point. Talk to parents who have children at the schools already.
Also find out if the schools cater for your child's interest, ie lunch time/ after school clubs. Most of the schools in our area do an open evening where the pupils and teachers get to show off their school and day time tours where you are taken around and see the children in lessons.