How are you certain of this?
If you have an overly-touchy manager, and he never overly-touches you, would you know?
If you have someone in sales who makes not-that-funny sexist comments, but they're not actually bad enough that you should bother caring, would you know if it's rising to the level of being a problem for women?
If you don't offer generous parental leave, and that matters disproportionately to prospective employees who are women but not so much to men, would you know? (Do you know, off-hand, what your employer's parental leave policy is?)
If one of your usual interviewers says something like "You're pretty good for a woman" (and not "You're pretty good for a man"), and your technical interviews are one-on-one, and this is why there's a drop between job offers to women and new hires, would you know?
(The answer may well be "yes" for all these; there are systemic problems far away from all these that can easily get a company to 10-20% women. But I, personally, am hesitant to answer "Is your company a good place to work for women" with anything stronger than "I'm pretty sure it is, but I couldn't personally vouch for it.")