Most "factual" questions can be answered by looking at their websites or googling such as the content/structure of courses, percentage of foreign students, employment x years after graduation, links with industry, costs/ranges of accommodation, etc., so perhaps best to research those areas in advance.
On the day itself, concentrate on things you can't get from the web, such as talking to students about the reality of student life, whether they've used the pastoral/MH support services, helpfulness of staff, quality of accommodation (cleaning, repairs, standard of beds etc), how much "in person" teaching time, whether there are small group/individual tutorials, etc. When talking to teaching staff, concentrate on content of the course or if you're undecided, ask about course options/alternatives, etc.
Pointless spending valuable/short time asking things that could be answered by googling!