Monitor the progress and performance of the girls vs the boys. Who is doing the best in your school? I suspect SLT do gender reports to the Governing Body but ask to see them.
Look at sports team success, music participation, clubs available and other areas where the girls might get less of a look in. Are they getting the same opportunity? What picture do you form of their participation and what they get out of the school? What do you specifically offer for girls? Or are they tagged onto what’s offered to boys?
When I looked at co-Ed senior school for DD2, in the private sector, I saw wet hockey fields but pristine rugby pitches. Sacred cricket pitches but no dance studios. We couldn’t find dance taken seriously anywhere. Are subjects with higher female participation, such as photography and other arts subjects, fully funded and supported or is it more likely engineering is supported? Money clearly wasn’t spent on some subjects in some schools. If this devalues your girls, look at what you could do to improve the subject facilities they prefer. (I know there will be some who think girls and boys are the same, but they do choose different subjects)!
How much do the girls participate in Young Enterprise, sports teams, music ensembles, drama, science clubs, trips abroad, etc. Are they marginalised? Do they get identical opportunities?
Do lesson observation. Are the boys answering all the questions? Do the girls sit passively? Are they asking questions?
Lastly, ask them. Do they feel supported and valued? Do they think they get the same education and attention as the boys? How are their concerns addressed? Gather some evidence. It could be interesting.