Along with her critical and popular success, there are many people who detract from the works of JK Rowling on a number of grounds. Some people feel that her writing style is too simple, although a simple writing style is not at all bad. Some people think that her books go into too much detail, and wrap themselves too much in their own history.
One of the more valid complaints against Rowling is that because her fans are well aware of her love of foreshadowing, misdirection, and surprises, fans may be thinking more about the intent of the author than the intent of the characters. We begin to wonder whether the characters are behaving that way from internal logic, or because Rowling wants to throw us some more curve balls.
However, it does make sense that Rowling would spend so much time confusing her readers, because it at least puts them on the same page as her characters. In the six books of the Harry Potter series so far, many different objects have turned out to be something different than what they seemed, and many different people have turned out to have different motivations than what was first guessed. In the earlier books, the mystery and reversal was usually handled in a simple way, akin to what you would find in a children's mystery book. As the books got more complicated, the revelations about thing's true nature got more political and shaded. In Book IV, Harry finds his impression of Cornelius Fudge, the Minister of Magic, wasn't quite accurate:
Harry couldn't believe what he was hearing. He had always thought of Fudge as a kindly figure, a little blustering, a little pompous, but essentially good-natured. But now a short, angry wizard stood before him, refusing, point blank, to accept the prospect of disruption to his comfortable and ordered world...
In short, almost everyone and everything in Harry Potter can be revealed to be something different than what the characters and readers expect. This is part of the thrill of reading the books. However, if JK Rowling decided to arbitrarily reverse anything and everything she did, it would ruin the story, both internally and externally. And, despite some on the Right's views, JK Rowling's morality seems to be basically conservative, and she wouldn't want to do a Grant Morrison style mind fuck on either her younger or older readers. There is some logic and consistency to tell truth from illusion in her books.
One of the biggest epistemological problems in the Harry Potter series, for both the characters and the readers, is the allegiance of Severus Snape, the former Death Eater turned either Double- or Triple-Agent. Throughout the books, despite many people who doubt him, Dumbledore continues to place his trust in Snape. In Book VI, Dumbledore continues his faith in Snape, even as evidence mounts. In the climax of the book, Snape seemingly murders Dumbledore, an event that has not at all ended discussion about Snape's motives.
While both many characters and many readers have opinions about Snape, and his possible history and motivations, it seems that Dumbledore has actual knowledge, and that this knowledge can somehow rise above the illusions and deceptions that seem to confuse everyone else. That there is a difference between appearence and reality seems to be the central epistomological point of the book. Yet, what is the difference between opinion and knowledge?
It is hard to make predictions about Harry Potter, but I think the revelations in Book VII will not be only about what Dumbledore knew, but how he knew it. While there will probably be a spectacular incident that reveals why Snape may be trustworthy, there is probably a deeper aspect than that. I also doubt that Dumbledore will base his knowledge on his technical skill in legilimency, or any other technical magical means. Rowling seems to hint in the books that Wizards in general don't have any special wisdom or insight into the world, and can be just as prejudiced and deluded as their non-magical counterparts. I think that Rowling has another idea where knowledge can come from.
At several points in the book, Dumbledore tells Harry Potter that Love is the one weapon that Voldemort can not use, and that it is more powerful than any type of Dark Arts. The exact tactical usage of love has even been demonstrated at the end of Book V, although what role it is going to play in the end run of the series remains to be seen. However, since love is seen as an offensive force, it could be that love is also a way to gain knowledge that is beyond just normal opinion. In other words, Dumbledore has knowledge of Snape and others because he loves him, and is able to see into his soul directly.
Whether this will be stated, and how it will be stated, remains to be seen, with the release of the seventh book.