How does Westerberg convince the police he did actually know who "Alex" was?

From the text:

“Well, by then I was kind of pissed at getting the runaround, so I told him, ‘Look, I’m not just another crank caller. I know who he is. He worked for me. I think I’ve even got his Social Security number around here somewhere.’” Westerberg pawed through the files at the grain elevator until he found two W-4 forms McCandless had filled out. Across the top of the first one, dating from McCandless’s initial visit to Carthage, in 1990, he had scrawled “EXEMPT EXEMPT EXEMPT EXEMPT” and given his name as Iris Fucyu. Address: “None of your damn business.” Social Security number: “I forget.” But on the second form, dated March 30, 1992, two weeks before he left for Alaska, he’d signed his given name: “Chris J. McCandless.” And in the blank for Social Security number he’d put down, “228-31-6704.” Westerberg phoned Alaska again. This time the troopers took him seriously.