Pick the one that best applies:
1. No you didn’t. The answer sounds plausible and you are a reasonably smart person so you quickly absorbed it as the correct one. So quickly, in fact, that in hindsight it now feels like you knew it all along. It is hard to have a memory of not knowing something, because way back when you did not know, you did not know that you did not know. So now you think you knew it all along, because you know it now and you don’t have a distinct memory of not knowing.
2. No you didn’t. You have previously wondered, or maybe just heard conventional wisdom that sounds like the answer you know now. Now that you know the right answer, the one you have just heard, you can search your memory and discover that you’ve thought or heard something vaguely resembling the answer before. But in fact, if you really thought about it, you could probably dig up a memory or some conventional wisdom that supports a completely different answer. Consider also that you never took a public stand, you never made it real, you never made yourself accountable for the answer you’re now claiming you knew all along. Which means that if the right answer had turned out to be completely different, it would be just as easy to say you knew that one all along instead.
3. No you didn’t. You thought it all along, but you didn’t know it all along. Your beliefs were based in your ideology or your worldview, not on any objective evidence. If you ever encountered somebody who believed differently because they had a different ideology or worldview, then at most the two of you stood there talking past each other, offering zero enlightenment to anybody approaching the issue without prejudice. Those people needed hard evidence, and you only had arguments. You didn’t know, you just thought you knew.
4. No you didn’t. You made a lucky guess. You are mentally engaged with the world, and so like all mentally engaged humans you form lots of guesses and speculations and opinions about lots of things. If you guess enough times about enough things, some of those guesses will eventually turn out to be right. That doesn’t mean you knew it all along.
5. No you didn’t. You knew the superficial version that everybody knew and that, to the scientists, was beside the point. The story you just heard or the press article you just read has omitted the scientifically interesting part. The scientists weren’t interested in the simple descriptive fact, the one that they, you, and everybody else knew all along. They were interested in how it worked or why it was the way it was.
6. Yes you did. Congratulations. You are hereby authorized to say things like, “Still no cure for cancer,” or “My tax dollars went to this?!?” Have at it.
The Hardest Science 
Amen. I like Stanovich’s (“How to Think Straight About Psychology”) example of this that there is plenty of evidence that having a job as a young person actually lowers your opinion of the value and integrity of work in general.
This makes sense to me; the kinds of work that teenagers tend to get are not likely to be contexts which would encourage positive attitudes towards work, but the opposite has been common sense for so long (“It builds character”).
This post is awesome. The next time I hear statements like “isn’t that obvious” or “isn’t that just common sense” or “I knew it all along”, I’ll think about this post.