How tall would a wall have to be to break Humpty-Dumpty?
It's not easy. You've got to sit down an think about all the vague exploits and hijinks that will happen, character details that you often forget to pay attention to, and all the while, you're sure that there's something that you haven't remembered.
Wikipedia's always a good place to start, but it's not a story-solver. After that, there's the Reference Desk on the Nano forums. People are more than willing to share their wisdom. Only problem is pretty much every other person on Nanowrimo is asking questions too. At that point, you might try wading through the different forum posts, which very often does turn up useful information.
Of course, if you've some VERY specific inquiries to make, you have to take on the Google beast or the labyrinth of your library. This is not for the faint of heart. In 2008, I was writing about a woman who had sustained a severe traumatic brain injury. I wanted to know what her chances of recovery were from different specific scenarios. It wasn't easy. A lot of the information I needed was locked away on websites that required $800+ dollars a year for access to their articles. So I spent the first half of October piecing together the information for myself using diagrams of brains and TBI patient resource sites.
You can make an inquiry with a specialist, or perhaps someone on Nano, but usually you're in a hurry. Come November, you realize what it was you really needed to research back in October. You're falling behind on word count, and no one's answering your Reference Desk post. None of scientific/medical lingo really makes any sense to your literary mind anyway.
In the end, you might even find you wind up leaving this information out all together. Your story might no longer make sense, but that's what revision's for, right?
Of course, when you do get the information you need, it can put your story together or break it to pieces. But even then, it's hard to be completely sure your story is believable.
It must have been a writer deluged in this sort of exhaustive research who first said: "Write what you know."
Not that, I think, anyone ever takes this advice for more than five minutes together.