Hi;
They are not actually allowed to ask stuff like that - why? I guess the most obvious answer to that would be that the dental schools aren't allowed to discriminate based on where you applied (and for that reason, they don't know where you applied).
Personally, I told them straight out about my other interviews. But I was careful not to indicate any kind of sway; I discussed the merits and shortcomings of the schools, and I avoided going into a great deal of detail.
I don't think there's a right way to answer the question, though. Just don't lie, and don't suck up, nor show any outright derision. I guess they want to see how you evaluated the schools you were at.
Anyone else have an opinion?