|A given post might be perpetuating sexism/misogyny if:|
It is dismissive of another poster in a way which would not occur to me if I believed hir to be male.
It indicates that there is a certain acceptable way for women to behave and a certain acceptable way for men to behave, which are distinct from one another.
It uses female-gendered language in an insulting, belittling, or diminutive fashion ("girls," "pussy," "hysterical").
It assumes that something about another poster's comment is due to their gender/identity, when this is not made explicit by the content of their words.
It assumes something about another poster's gender/identity based on something about their posting style or position, when this is not made explicit by the content of their words.
It perpetuates or relies upon stereotypes which pertain to gender.
It has the effect of shouting down the opinions of female-identified posters without the courtesy of listening to them with an open mind first.
It has the effect of speaking for female-identified posters without the courtesy of listening to them with an open mind first, and giving them the opportunity to speak on their own behalf if they choose to.
It otherizes women as exotic strangers which no right-thinking (male) person could hope to actually understand— rendering futile, by implication, the act of listening to women.
It demonizes women who speak up about their opinions as shrill, negative, overreacting, man-hating, bitchy, or hormonal, which tends to have the effect of silencing them either literally or figuratively.
This is by no means an exhaustive list.
Like posts which contain coded racial prejudice, posts may contain gender bias without rendering the poster Irredeemably Evil. Who was it who offered the analogy of trying to throw a paper ball into a trash can and accidentally hitting someone with it? It doesn't make you a bad person, or weak, to say you're sorry and will try harder not to hit them next time. But if one keeps getting hit with paper balls by someone who refuses to apologize, it's within reason for one to assume there's some intent behind it.