There are a series of canned questions that should always be asked upfront, and these should be documented in the requirements. However, I also find it essential to ask the business to provide a mockup of the end product. Let them actually draw a picuture (preferably in Excel with real numbers that add up) of what the report or dashboard screen fed from the datawarehouse should look like. This Excel sheet is something that should be passed around to all the stake holders and end users so they have an opportunity to input. As a reality check, compare that mockup back to the original requirements document and you'll be amazed how many critical holes it fills, especially if the requirements were written by a 3rd party or were pre-written before you were hired on the project.