1) Do you do exiting interviews?
If they say no, that's a huge red flag. Even in a small company, asking someone "why are you leaving?" isn't hard and doesn't require a tremendous amount of resources.
2) Following up from question 1: What do most people state as their reason for leaving?
I've never met an interviewer that could rattle off statistics like "last year X% of resignations were attributed to issue Y" - but I'd be seriously impressed by one that could. Retention of talent is a huge issue for employers right now, so if your interviewer is in management they should have at least a vague idea what the answer to this question is.
3) How would you describe your company's culture?
Every answer you get to this question will be biased. It's virtually impossible for anyone to objectively measure culture, and it's even harder to do from an insider's perspective. At best, you'll get an answer where you have to read between the lines. At worst, they'll totally bullshit you. If your bullshit detector is finely tuned, then this can be a very revealing question. Either way, the resulting discussion is often more illuminating than the answer itself.
4) What's your intellectual property policy? Can I work on open source projects on my own time?
I've received answers to this question that span the full spectrum from totally incompetent ("I have absolutely no idea") to totally absurd ("we own every line of code you write, regardless of context").
I feel you can get a sense of the vibe from talking to the interviewers, seeing how they act, what they say, and how they are leading the interview.
However, sometimes you don't know about the culture until actually working there, so it is good to try to interview them while they're interviewing you to make sure it's a place you want to work.
Granted this is for the IT sector, and you should always checkup on the company in more depth to ensure it's completely the right fit
That's how I landed my current gig. Product designer at one of Silicon Valley's hottest startups. True story :)