Show an interest in graduate students.
Who are they working with? What research are they doing? What do they like about their research, their mentor, the program? What don't they like? One-on-one conversations can be very helpful. Also pay attention in group settings. How do the grad students interact? Does it seem like they're friends? Competitors? Watching their interactions might offer insight into the social side of the program.
Don't underestimate the potential influence of current graduate students.
Although they might sound like they're speaking off-the-cuff, remember that you're on an interview, regardless of with whom you're speaking. Present your best side because current graduate students may be in a position to help or hurt your application. Some interviews include social events like parties. Don't drink (even if others do). Remember that even though it seems like a party, it's an interview. Assume that you're being evaluated at all times.

