Culture is a difficult thing to discover in a relatively short interview, but usually I look for certain signs. Typically if I feel that the interview is more like an extended discussion instead of like being in a game show, it's usually a good sign.

It's also things around the interview itself, e.g. are you being treated respectfully, or do you feel like being just one CV among many. Do they respect your time; do they talk as an equal, or as a superior person; do they get back to you if they don't go through; and more. I find that a company that treats their applicants respectfully typically have a more agreeable culture.

Of course this is just my experience. Some game show style interview companies may turn out to have a great culture, but what I find without fail is that if you're respected as an applicant, the culture can't be that bad :)