If you want to avoid sugar, aspartame, trans-fats, MSG, or just about anything else, you read the label. If you want to avoid G.M.O.'s -- genetically modified organisms -- you're out of luck. They're not listed. You could, until now, simply buy organic foods, which by law can't contain more than 5 percent G.M.O.'s. Now, however, even that may not work.