There is a search bar in the upper right. I bet if you look up questions to ask interviewer, or something similar, there would be many threads that have already talked about this.
Other than that, pull from your shadowing experience that you hopefully have. Did you see any CEREC? Are you interested in doing research in dental school? Do any students get to place implants? Specialization placement? Board pass rates?
Don't ask questions just for the sake of asking questions.