Once you have been admitted, the department will typically help put you in contact with graduate students. Of course they will select the most enthusiastic ones, so it can be helpful to reach out to others to get a better sample (for example, students in your research area or who are working with potential advisors). You can generally find contact information for graduate students on the departmental web page or on their personal web pages, and it's reasonable to send a quick e-mail. You can ask whatever you'd like, but you should remember that it's not really their job to tell you about the department. It's best not to ask questions you can get answers to elsewhere (for example, don't ask about degree requirements), awkward questions (such as departmental gossip they might be uncomfortable discussing, especially in writing), or repeated/demanding questions (after all, they are doing you a favor).
On the other hand, I don't think you should contact graduate students before being admitted. Most graduate schools receive substantially more applications than they will be able to accept, and sometimes far more. It's not efficient to spend a lot of time telling people about the department if they are probably not going to be admitted anyway. Instead, that can wait until the admissions decisions have been made. (Plus you'll generally get more enthusiastic responses once you've been admitted.)