1. "During your most recent budget period, what influence did politics rather than the merits of budget requests have in distributing resources, and in what ways?"
2. "How heavily influenced by politics (or upper management) is the performance review process?"
3. "Has anybody recently been let go as the result of being ranked lowest on a performance review?"
4. "Are managers subject to performance improvement plans?"
5. "When was the last time your HR department found in favor of an employee over management or upper management when a dispute arose?"
There's a bias to these (if it isn't obvious). But experience informs me that answers to these questions offer a good insight into how the company really operates. If they are honest answers (they won't be, most of the time, if they're answered at all).