It has to be as objective as possible. One thing you could do, is measure screentime of the genders, like someone said. But if you do it like that, you have to control for confounding factors or other factors that can partially explain the results. Maybe men go more often to the cinema (watch more movies), which means it makes sense for movies to be catered to men, which might result in less female characters on-screen. Or since men are more risk-takers and known as such, more movies have male (main) characters, because adventure/action movies might do better etc. Also, I don't think that women/men must have 50/50 screentime. I'm more of the equality of opportunity viewpoint, and less of the sexist equality of outcome viewpoint.

Now about sexistly (not a word) written characters. What is a sexist character? One that follows a trope ('damsel in distress')? Than the same goes for men on the flipside of the same situation ("knight in shining armor" e.g. whenever a woman is kidnapped, a man has to save her, risking life and limb, killing tens or hundreds of men, because that one woman is much more valuable than all those men, including the 'knight' himself). A feminist might argue that 'damsel in distress' is harmful and 'knight in shining armor' not, or less so. Then the feminist may prove it, with an objective, scientific study. Until then, we'll assume that it isn't the case.

I don't think that women are 'underrepresented' in movies and I don't think that women in movies, are portrayed unfair or sexist, generally speaking. Definitely not more than men are. If so, I would need to see an objective test that proves this. And it's hard to make a test, because what is sexist is subjective to the observer. Is scantily dressed 'expressing sexuality' and empowering women, or 'fucktoy for male pleasure'?