Well, he is famous for reading through the thousands of pages that used to compose the "Moodies" as an extremely young man... Now, in the latest interviews that I saw of him, he explained how he basically reads any 10K report that he can get his hands on...
By the way, to elaborate just a little bit on your question which seems incomplete to me... Actually, I think that more important than determining the provenance, is to understand that Warren has a very personal way of interpreting the numbers that he'll come accross on any companies.
Indeed, throughout all his career, he has basically gone from searching for the cheapest companies to buy, to searching for the cheapest moment to buy a very valuable company... There are many keys to calling a Company valuable, but the key one of course, is the predictable preservation of a very well defined durable competitive advantage... The capitalist "game" being predictable in this way, any long-term investor will need to have a clear understanding of what can be exploited by competitors, and what will by definition, never be anything else than a single company's advantage...
More globally, from my perspective, he and his even more remarkable perhaps, right-arm Charlie Munger, have the very rare capacity of objectively assessing of the situation of any companies. A lot of it is psychological strictness... The magnificient 1995 talk in which Charlie Munger explains what he calls the psychology of human misdjudgment is maybe the best illustration of what I try to describe here... An absolutely key video which I think necessary to watch...