Academic results and progress 8 scores are a good indicator but can be skewed if you live in a grammar area.
Ofsted can change and outstanding schools are often not fully inspected over long periods so lots can change but usually good or outstanding would be most parents preference so over subscription can be an issue.
Word of mouth is generally ok but one child's experience can vastly vary from another child's so speak to a number of parents to get a general idea if possible.
Ask on local forums or community pages about local schools, obviously you will get a mixed bag of responses and not everyone will have a glowing report but it can often help if you have no friends of older children to ask.
Visit as many open days as you can (they are often advertised in the local free press or on school websites) Open days are good for assessing available facilities and asking specific questions about class sizes or asking about particular specialisms etc They are also a good opportunity to speak to current pupils however often only the well behaved pupils are asked to help out at these events. Don't be swayed by a glossy brochure or a rousing headteacher speech alone as a lot of school trusts now have marketing departments.
Check admission arrangements for each and every school and work out what "category" your child's application would fall in to. Contact the school and ask if the school is over subscribed and if so which category it cut off in and at what distance if applicable. Remain realistic about the likelihood of a successful application.
Use all your preferences, don't just pick one school it will not move you up the list in any way. If you are allocated a school you do not wish your child to attend do not refuse the place offered as the local education authority has fulfilled its legal to provide a school place and is not obliged to find you another. Hold tight appeal for as many schools as you want, put child's name on waiting list for each preferred school and ring the lea every couple of days to see if spaces open up in other schools that you might consider.
DO NOT miss the deadline for school applications as late applications go to the back of the queue irregardless of admission criteria position.