Second (and I kind of hate to give this one away), I ask "what do you sacrifice when recruiting to ensure diversity?" The answer should be either "it takes us longer to recruit, because we ensure a balanced pool" or "we have changed our process to allow many types of candidates to shine". If the answer is just "we spend lots of money on sending people to Grace Hopper!" it means they aren't willing to inconvenience or piss off overrepresented engineers. You can't fix culture problems by spending money, and when "diversity" is seen as separate from "recruiting" in general it's a clear sign of a problem.
I also use the Internet: I look on LinkedIn for people who have left the company, see how long women stayed and reach out to find out why they left if they did. Backchannel mailing lists are great ways to find a vouch. I check the social media profiles of their prominent engineers and search with keywords like "feminism", "women" and "she". See how they talk about women in the field, whether they follow women on Twitter, whether they posted angry anti-Hillary memes. I've found that a lot more effective than relying on direct questioning, because there are companies out there that will say whatever they think will let them add you as a shield against the accusations of sexism they are facing. Especially my boss: if my boss is going to be a white dude, he had better have publicly condemned sexism in a way that feels constructive and genuine to me.
One other internet trick is to look at where the women you respect are working and apply there. At the very least, you would get to work with technical mentors you admire and can learn from.
You can find good spots here. Seriously, though, trust your instincts, don't be afraid to walk away and don't be afraid to take an "unsexy" job at an old-school company with an HR department.