Re: whoever wrote this really hates women, what are some of the more frequent red flags you've seen?
Okay so this was based specifically off of reading a fic where two different canon female love interests were written off as "cheating bitches" despite neither of them being unfaithful to the male love interest in canon, so...that, for one thing.
A non-exhaustive list of things that make me think the author really doesn't like women that much:
Women regularly being described using derogatory language (bitch, slut, etc)
Major female romantic rivals being written off as cheating, abuse, gold-diggers in an out of character way (for fics)
Female characters being described as shrill, hysterical, etc
Male characters being written in negative ways by giving them traditionally/stereotypically feminine traits (e.g., overly submissive, especially to an "overly assertive" female character)
Fridging female characters
Relationships between two women (usually two cis women, when genitals are relevant to the conversation) being erased or existing only as an afterthought while m/m relationships are a major focus
Female characters being derided or "getting their comeuppance" for being popular, being attractive and aware of the fact that they're attractive, liking/caring about fashion, being "overly" interested in men, etc
"Not like other girls" narratives
Erotic descriptions of women's dead bodies