Q: What should I do to let the program know I am interested in the Shomer Shabbos spot? A: Do not bother them on the phone about it pre-interview day. When you get to the interview day discreetly ask either the residency coordinator or program director. Even the person who is interviewing you is a bad person to ask as they will likely not know enough to tell you anyway and the fact that the Shomer Shabbos possibilites at the program is a draw to you may not come across as a positive to the interviewer. There are many program directors that do not want to hear about Shomer Shabbos spots and you can kill your overall chances at a spot there by asking so know your audience.
Q: What information should I gather?
A: Ask about how they handle the major holidays and the minor holidays (don't be too hopeful on Chanukah or Purim), Shabbos with regard to: call, night float, when winter comes and it starts early, kosher food, local synagogue (in hospital?), how the other residents feel about this different treatment etc. You would do well to point out that you want to work just as hard as anyone else in the program, and you want to be sure the other housestaff knows you are working hard as well.