Lucyccfc:
I heartily agree with what MillyMollyMama posted. Open Evenings/ Days are usually highly orchestrated and about putting your best face forward to prospective parents.
We also talked to students who were scattered about or guiding us to places - and we specifically asked about sport club/ team opportunities as DD1 (now Year 7) is sports mad.
The academic side (if that's your concern as a parent) can be resolved by statitistics - have a look at Parent View results for the school / but also look at the run of GCSE results over the last few years. Also - check out the school's website & newsletters: is there a lot of information for parents? Do you get the impression a lot is going on there?
The social side is important - try to get an impression from your child about what schools are considered good/ nice by other kids. I did a lot of asking parents/ DD1 - which school is x or y considering? Don't underestimate the 'where are your friends hoping to go' factor.
Also consider the commute to school: nearby? can you drive them there? do they need to use public transport? journey length? location: quiet streets/ busy roads? Are you comfortable with your child travelling on their own on public transport?
If you know parents who have children already going to a prospective school - talk to them: how is communication?/ is the school relatively organised?/ is it caring? encouraging? supportive? If it is a concern: ask about how they 'set' or 'stream' students in subjects and what they think of the work load: are subjects moving fast enough? are targets ambitious? is there regular homework? etc....
Finally - know that there is no 'perfect' solution to school choice. It's always a bit of a compromise - but remember it isn't just down to the school. We have good friends who sent their eldest to a school which is now in special measures and has a poor reputation locally. For them it was about the commute - because they have younger children at the nearby primary. I know at the time they were really concerned they'd made a horrible mistake by putting that school top of the list of comprehensives (the girl just missed getting a place of local state funded grammar schools).
As it turned out, this girl absolutely thrived at this very run of the mill secondary school - top set in all subjects. She was supported/ encouraged/ given extra work/ etc... - went on to get all A*s at GCSE and has transferred to a state funded grammar school for sixth form. On paper, academically this school didn't appear to be a great choice - but in reality it was an ideal environment for her. She was a real star at the school - the teachers bent over backwards to encourage/ support her - and probably were even more thrilled for her GCSE results than she was.
HTH