I think there are a number of questions you must ask each other before marriage. But the most crucial ones are related to:
Money: How will you handle finances? What are some thoughts on savings? Will you have a joint account?
Children: Are you both interested in having kids? How many? And how will you raise them?
Location: Where will you both live? Are you both willing to relocate?
I believe these 3 aspects are the most important to discuss before marriage.