We believe that experienced users hold a great deal of the semantic intent of a system in their heads, but of course it has not been organized in any usable manner. To ask a user "What are the semantics of your system?" would be even less productive than to ask the user to design it.
We have conducted more than 100 semantic modeling elicitation sessions with users and analysts in dozens of industries. We can generalize several things from this experience and suggest several techniques for making it more productive. I have summarized the recipe behind the primary approach as being three parts anthropology (talking to the natives) and two parts archaeology (digging through the ruins).