"Reporting" is the act of telling the details of a news event. "Interviewing" and "investigative interviewing" are the means by which the facts pertaining to events are gathered so they can be reported. The original wording of the question was "went out and did some reporting" when the publication itself is the reporting. What is meant to be asked is what indicates that Wolfe went out and did some in-depth interviewing so that he could report it.
The first best example of Wolfe's actual participation in interviews resulting in reporting is found in "The Truest Sport: Jousting with Sam and Charlie." Part of the technique of New Journalism, as practiced by Wolfe and some of his contemporaries, is "saturation reporting" whereby the interviewing reporter "shadows" the subject for hours to days to weeks (Hunter Thompson spent 18 months with the Hell's Angels to write a book about them), then reports the dialogues, scenes, settings, emotions, reactions, etc of the subject that are uncovered during the saturation period, which are then reported in news reports, essays or nonfiction novels, like Capote's In Cold Blood.
Saturation interviewing technique is evident in Wolfe's reporting of Dowd's life as a fighter pilot aboard an aircraft carrier from the first moments, when Dowd is awakened by the officers' steward saying "Bye borty-bibe," to the screeching images of the fuels and flames and fury of the F-4s as they launch or are "rescued" in landing.
Without real on-the-spot interviewing, these scenes would be no more than impressions of an active imagination and would qualify "The Truest Sport" as historical fiction rather than New Journalism's nonfictional reporting. Thus the detail on every level is a clear demonstration that Wolfe "went out and did" saturation interviewing for nonfictional reporting of a worthy series of events and worthy personalities directly involved in the Vietnam War.
To say that an F-4 is coming back ... at a speed of 135 knots ... that may be the truth on paper, but it doesn't begin to get across the idea of what a man sees from the deck itself ... the plane's speed does not diminish ... it's a brick, and it's not gliding, it's falling ... [headed] for me ... smash! it hits the skillet....