You don't. Technically programs are not allowed to ask you this in your interview but many do anyway. The only way a program will know if did a rotation somewhere is if you get a letter from one.

To be realistic: I am interviewing right now and at EVERY single program (6 so far), at least one of my interviewers have asked me where I did each of my aways.

Yes, I think in regards to what both people said above: when you're applying, beyond who wrote your LOR, nobody will know where you did your aways. However, once the interview comes, just about everyone asks where you did your aways, and most have asked why you chose those and what you liked/disliked about them. Choose aways wisely, as you are more likely to end up at a place you rotated than anywhere else. Hope that helps.