An informational interview helps you understand the bigger picture so that you have better information to make your decisions. See the extensive Guide to Informational Interviewing from the Career Development Center. It will guide you through finding alums, the polite way to contact people, and also suggest a host of other questions you might ask, depending on the type of decision you are trying to research.
List your potential majors.
Find at least one Stanford alum who majored in one of those.
Contact them for a chat and ask how they made that decision and whether that major has affected their career post-Stanford.