If it's a private tour with a staff member at the school (i.e. someone whose job is to sell to school, who gives presentations to pre-vet clubs at other schools, etc.), they are very used to all kinds of questions and know all of the ins and outs of the school, and about the students. My school's guy will give you a 3 hour tour (without being stranded on an island) if you are up for it. He loves to talk and will tell you all the stories he can, so questions are welcome at any point. Ask about interesting cases, typical caseloads at the hospital, what kinds of extracurricular activities there are, how involved students are in those activities, if there are any opportunities to get hands-on experience before clinics, if they welcome students to shadow clinicians in pre-clinical years, etc. I didn't ask many questions on the tour at my interview, but I did chat the students up about classes and the possibility of working while in school.