I'll start by saying that genetic modified food isn't necessarily a bad thing, although it can be. I'll leave that for another post. This post is about price lookup codes, or PLUs found on products in grocery stores. While most of them are 4 digits in the 3000-4999 range, if it's prefixed with a fifth number, a '9', it's organic. If it's prefixed with an '8', it's genetically modified.
UPDATE 12:22pm PST: This article by food-ethics.com suggests that using the '8' code is voluntary, kinda like registering a .xxx domain name, so in practice no one uses it.