The first step is to let it sit for a while - if it still seems like a good idea after a couple of weeks, then it probably IS a good idea.
The second step is to really examine it and experiment a bit. Try to sketch a couple of pages, poke at any potential plot-holes, try to figure out the plot-twists, etc. If it holds up, it's a good idea! If it doesn't, it might need to sit a while longer. You don't have to throw it away - it might just need more time.
Grassblades needed, like, 4 years before it came together. ^_^;