Two skills have served me well while gathering information about an enterprise. First, the lessons it has been my pleasure to have learned from my customers and the many, many, bright practioners I have been lucky enough to work with. Second, the ability to forget all of those lessons and question everything.
When first going on an architecture safari the goal is to trust but verify. Even when working with knowledgeable guides, there are long stretches of boring research punctuated by sudden blasts of uncontrolled detail. Information seems to be everywhere. In the volumes of arcane regulations lounging quietly on the cubical shelves, in the patterns of worker ant like activity which continues as though you didn't exist, as well as the sharp witted proclamations bestowed upon you by high powered consultants who, after years of dedicated service have yet to deliver but are all too willing to tell you why they are failing. You need to take it all in, putting each item in context with its source and related each item with others to decide how much impact it will have.
Don't get me wrong, generally people really do mean to be helpful. They speak authoritatively; blissfully unaware of their own errors of omission. Although I find they are more likely than not going to try to TELL you the answer... with little regard for your actual question.
I have found myself working on my interviewing skills to address this phase of enterprise architecture. For the friendly sources I have taken to watching body language and seeking to engage them using their own words ... their terms not mine. Of course not all sources are friendly and those hostile few are generally more valuable, but care is required to manage the conversation. In these circumstance I find interrogation techniques used by law enforcement very practical.
Throughout the information gathering process, without regard for source, understanding is more important than knowledge. Focusing on the business operations, not the activities of the systems, and seeking root cause for each issue identified have proven to be absolutely necessary. The need for more information never ends but your hunt for qualified, trusted, useful information will be time-boxed.