Five years ago, I would have said that the percentage of women in an office isn't necessarily a good indicator of anything. Nowadays, this would be my first advice: ask how many women work there.
With maybe one exception, all the places I've worked in that had very few women were terrible places to work in. Most of them were unpleasant to work in even for men who think "bro" is not a word to be uttered after you turn 19.
Teams that have a strong bias against women act on it almost universally: they drive candidates away with shitty and/or unenthusiastic interviews and they make life hard for those candidates who do get through. They don't end up with all-male teams just because reputation preceeds them and no woman wants to work there -- they end up with all-male teams because prejudice and insecurity tend to tip the balance of their hiring decisions, too.
It's not a universal predictor, but I definitely consider it a red flag. Frankly, it's one that I look at, too. I'm not the SJW type, but when I got into this whole programming thing, hacker communities used to be inclusive and diverse, and I kind of like to keep that going.