Ask your customers to bring examples of artifacts created or modified by your product or service. Ask them to tell you why these artifacts are important and when and how they're used. For example, if your product is a software system to manage invoices, ask them to show you the invoices, reports, or spreadsheets that they've created through using your product. If you make running shoes, ask your customers to bring you several pairs of worn shoes and tell you about all their runs.
Pay careful attention to anything that surprises you. What did you expect customers to create or modify that they have ignored? What things can you do with your product or service that aren't used? What was used in unexpected ways? What do these tell you?