Directly behind some of the flowers, you will begin to see the courgette forming...these are females = good

Some of the flowers will just be on elongated stems, these are males.....they don't have the 'roundness' of the females.
Hope that has described it adequately......you'll see the obvious differences when you look

Just pinch off the male flowers and discard before they open up....like I say, if they pollinate; the fruit will be very bitter.
They are very, very prolific producers.....pick the fruit nice and young and they will just keep comming and comming.
Get a recepie for rattatoulle ready

ETA - just remeber, some vars are feminised; so you won't neccessarily get male flowers......better to keep checking though, as even those can throw out males (where have I heard that before haha). You soon see the differences in the flowers. ATB