I've got another reader question about writing that's pretty timely, considering this is where I am right now in the writing process: Can you tell when an idea is powerful and has the potential to be great, and when it's just a silly idea that probably isn't worth developing, or do you need to develop the idea to its limits in order to know?
On a purely non-scientific level, I often judge ideas by the "tingle test." The best ideas are the ones that make me stop and go "oooooh!" and I get tingly all over. It's like time comes to a stop. I remember that I was three steps from the top of my staircase and froze when I got the first inkling of an idea for what became Enchanted, Inc.
But that sort of thing happens with a lot of ideas, not all of which pan out. The lasting ideas are the ones that haunt me, that I find myself thinking about at odd and random times, or that take over my brain to the point I can't really concentrate on what I'm trying to work on.
On a more methodical level, you can put an idea to the test by doing the usual things you'd do to develop a book. Think about what characters would be involved in this idea. What conflicts are inherent in the idea? What kinds of things might happen?
However, not being able to think of characters, conflict and events doesn't mean it's not a good idea. Books generally don't spring fully formed from your head so that you just have to transcribe the first thing that pops into your brain. The idea is only a spark. You'll probably have to do some work to develop even the most brilliant idea. You'll have to develop characters, determine their conflicts, plan events, etc. You may even need to do research on related or relevant topics. (Mind you, this probably only works for those who are plotters. If you're someone who just writes without any planning, I have no idea how things would work. You probably just have to start writing.)
I sometimes think of my brain's "idea center" as like those hopper things they use to select lottery numbers, the ones where the balls pop around like popcorn until one comes flying out. I have all these ideas or idea fragments flying around in my head. Every so often one comes to the surface, where I play with it a bit and develop it a bit more, and then it goes back into the hopper. Sometimes two ideas will collide and merge into a bigger idea. When it's time to write the book, the idea will come flying out of the hopper like a winning lottery number. For instance, when I first came up with the idea for my series, what struck me was the thought "Bridget Jones meets Harry Potter" because I wanted something with the magic of the Harry Potter series, but in a more adult setting, with the working world instead of school. That was all I had for a while. One day when I was playing with the idea, I thought it would need to take place in New York. I had another idea fragment in my head that was inspired by my first trip to New York and the way I responded to the city, with the sense of being a small-town southern girl in the city. That seemed to fit this idea. Then later I was thinking that my small-town girl would have to learn about magic somehow. The standard fantasy plot is that she would discover she had magical powers, but I thought that was kind of a cliche and it would be more interesting to maybe go the opposite direction and have magic not affect her at all. Still more facets of the idea came to me when I started doing research, and then more when I was discussing the idea with an editor. It's one of the best ideas I've ever had, but the initial idea took a lot more development before it could become a book, and it was more than a year and a half from the time I got the initial idea to the time I started writing the book.
So I guess the bottom-line answer is that there's no sure-fire way to know an idea is good without doing quite a bit of work. In general, I'd say the best ideas are the ones that excite you because then you'll be willing -- even eager -- to do the work that it takes to develop them to their maximum potential and you'll be excited enough about the idea to stick with it until you've turned it into a book. You probably have something if either you're willing to spend a year thinking about the idea or if you can't wait to start writing it -- and then actually finish it instead of losing interest. That's the real acid test of an idea for me, if I start writing it and then find myself not caring all that much.
Something else to consider is how unique and marketable your idea is. Take a look at the appropriate section of the bookstore, browse Amazon, do a Google search or follow the deal reports at Publisher's Marketplace to see if there's anything on the market that sounds like your idea. But don't get too excited about your idea being either too similar to existing books or totally unique. As long as your story isn't exactly like something else, especially something recent and famous, it's mostly about execution, and a really interesting take on even a tired subject can be successful. At the same time, if there's absolutely nothing like it out there, that could be a sign that there's no market for that thing. On the other hand, you could start the trend. The important thing to find out is whether or not there's something too similar on many levels, how recent it was and how successful it was. Something that bears a striking similarity in too many ways to something recent and famous or to something that just about every publisher has already published their version of probably won't be very marketable at this time. Ditto with something along the lines of something that recently bombed spectacularly (but give it time, things will come back around). The most marketable concepts are those that put a new twist on something familiar that is currently successful, but don't get too hung up on how marketable an idea is if it's really haunting you because you never know, and it really is mostly about the execution.
It would be nice, though, if there were some kind of idea-vetting flow chart or Internet quiz that could tell you right away whether or not the idea was a good one. I've gone so far as to write a whole book proposal before realizing that the idea was fundamentally flawed (and that I didn't care what happened to any of the characters).
Now, any more questions about writing?