One of the frequent questions we are asked is "how can I get the time to explore needs when the business says that it is already done - "it's in the business case", even when it is obviously not complete? This is actually a pretty common occurrence - nobody wants to take time to understand needs because they think it has already been done.
The best way is to integrate needs analysis into the way that you discover requirements. As new requirements (and I include user stories as kind of requirement), make sure you understand what the need is behind it. Use the questions I mentioned in the webinar (Getting to Needs: Seeing beyond wants and wishes to find the real source of business results) to expand your knowledge of why they "need" the system to work that way, and offer alternative solutions as you see them. The offering of alternatives will stimulate a discussion that will either confirm your understanding of the need or help you refine your understanding. If the alternative is actually a better way of solving the problem you will demonstrate your understanding of the need as well as help to build a reputation for adding value that will in turn improve your ability to collaborate with your business partners.