The ninety minute Triton Tours are honestly the best way for prospective students to get to see campus. Afterwards, you can explore campus on your own and check out Sixth. Talking to one of the Sixth Res Life administrators could get you a tour of the dorms, but keep in mind that people live there.
http://admissions.ucsd.edu/forms/tour-form.html
If you go on a weekend or during campus break, you won’t really get a feel for the school atmosphere. It might help to sit in on a class:
http://admissions.ucsd.edu/tours/ucsd-for-a-day.html