Use paper so that mking changes is not a problem. Sketch every report, including the screens as well as the paper printed reports. Get their opinions. Redraw the reports and show them to everyone again. All reports have a comparison.
Explicit comparisons are obvious.
Reports with implicit comparisons are things like mailing labels or invoices. At this point you make a decision about the availablility and suitability of turnkey packages. Next, draw an Entity-Relationship Model. This is a technique used to very specifically determine what the things are which need information processed. ER modeling is used in all sorts of enterprises, including enterprise analysis.