I guess it kinda depends on the facility. I got a tour of the TRACON and tower here at ROA in Virginia. I simply called the airport manager who in turn directed me to the tower secretary who scheduled it. I personally was surprised that they let us go on a tour because just a year earlier, they had said, no. That being said, ROA is only a class C airspace (and directly across the country
), so, it might be harder to get in on NORCAL or Oakland center. I would start with what sykocus said, never hurts to ask, they can always say no.
hope you are successful!