Feminism is defined as belief in and advocacy of the political, economic, and social equality of the sexes. So, if we believe the sexes are equal, is this something we can test empirically? I'd say it's not, at least given how the belief is currently expressed.
If feminists do believe in gender equality, then they often shoot themselves in the foot by arguing for ways that the sexes are clearly different. For example: men being stronger, women making a greater sacrifice in childbearing, men being more violent, men committing more crimes, women being more risk-averse, etc. And these aren't just trivial differences either, these traits are all either better or worse to have in aggregate. There are exceptions, like strong women and weak men, but is gender equality just the belief that there are exceptions? I'm sure you could find plenty of raging sexists who will agree that some women are stronger than some men. If sexism is about making predictions about individuals (like, predicting how tall someone will be), well then knowing aggregate measures and their sex might actually be useful for that. But if we know the individual well enough (like, how tall a dude is), I think even sexists would say we don't have to use the aggregate measures anymore in that specific case.
It's not just that "men are more violent than women" is a belief that could lead to sexism. It seems like it is the sexism. What is a sexist belief, if not a belief that implies one gender is better in some way than another? And let's say it's empirically true. If that's the case, shouldn't we update our beliefs and become sexists? Maybe gender equality is saying that even if men and women are different in measurable ways and some of those differences are clearly good or bad, women and men are equal in some non-measurable way (i.e. men and women who are equally strong, equally violent, and also controlling for everything else listed, are equal). But that seems pretty vacuous. Is any sexist really arguing that women and men are unequal in some "vague, unmeasurable way"? They're usually saying "men and women aren't equal due to x, y, and z differences." So obviously if we ignore those differences then we shouldn't believe they are unequal for no reason.
I think the normal feminist response would be to say that, sure, we believe the sexes are in aggregate different in some ways, but these ways can be backed up by evidence and statistics, unlike sexist beliefs which are false.
So if a belief must be false to be sexist, then "sexism is a lie" sure seems like a vacuous truth. Obviously something that must be false be definition will end up being false. If we were going to "test" sexism, then we'd have to take some belief that seems sexist (like "[gender] is on average [worse] as measured by [x metric]") and then see if that's true. But if it is true, we just update the definition of what we're testing so that gender equality is compatible with that belief. Let's say some sexist shows up and says "men are better at playing the cello while dancing." And we call them a sexist. But then we do a hundred studies and find that on average, men are better at playing the cello while dancing. Will we decide we were wrong and gender equality is a lie? Or will we just change the definition of gender equality?
But let's ignore the "belief" part of gender equality and move to the advocacy part. Men and women should be treated equally, right? Except plenty of gender equality groups would say there are exceptions to this, like women-only spaces, or building more women's shelters, or saying sexism against men shouldn't be as taboo as sexism against women because of societal circumstances. Aren't these just arguments that, in some circumstances, men and women shouldn't be treated equally? And what exactly distinguishes this? Is there any sexist policy that, if found to be a good policy, would prove that gender equality is a bad idea? Or would we just retcon what gender equality means? And if someone says or advocates something sexist, well, how do we know it's not just another exception? Plenty of other things that seem sexist turned out to be good enough ideas that even gender equality (feminist) groups are advocating for them, right? Maybe they're intended to make society more equal or just long term, but isn't that the stated goal of any policy, sexist or not? To improve society in some way, based on responding to the current circumstances?
I think the closest you'd get to a non-vacuous definition of gender equality would be someone who was literally against any policy that treated the sexes differently, or, if they had exceptions, was very clear about what the boundaries are around the kind of equality they're advocating for and willing to bite the bullet in some cases for the sake of gender equality.
One visible distinction might be collective responsibility. I think most proponents of gender equality would agree that men aren't responsible for the actions of other men and women aren't responsible for the actions of other women; it's not like they would make meaningful exceptions to that premise. But this is just one belief, so I don't think it's sufficient to explain which policies/beliefs fall under gender equality and which don't.
Obviously, there are plenty of sexist beliefs and policies that are bad (limiting property ownership or voting rights, or even male-only conscription I'd argue), and I'm cherry picking a few potential exceptions to make a point about categorization. But the point is, even if feminists make good arguments, many of them aren't arguments for gender equality. I can say that "limiting property ownership or voting rights, and male-only conscription" are sexist, but then some gender equality proponent could come along and say, "well this war is an exception, so actually male-only conscription isn't sexist in this case." The definition just changes based on what "makes sense." To some extent, it seems like both sides agree that gender equality isn't true as an absolute, they're just in disagreement over the details. And that's not enough to distinguish them from sexists, because sexists would probably agree that there are some ways in which the sexes are/should be different and some ways in which they're similar.
This isn't me saying that what people today call "gender equality" today is wrong, per se, just that what they're really advocating for is gender equality in cases when it makes sense, and at that point you may as well simplify things and just say "I support laws that make sense" rather than "I support gender equality." (If your position is "we should do x when it makes sense" then people are gonna agree with that premise, regardless of what x is.) And because people disagree on what "makes sense," this isn't even a rigorous definition. Even sexist people will probably support equality in cases when they think it makes sense, the real hurdle is convincing them that specific policies don't make sense, not that gender equality as a vague concept is good. To some extent this also applies to concepts like fascism/authoritarianism/capitalism when the real debate is often over specific policies but people seem to think it's over definitions or broader philosophical absolutes. There's a reason I didn't make this post about racism, but a similar definitional issue occurs there.