Since the government hasn't required the food industry (in the US) to post this on their labels it isn't easy to find out.
There is an app you can put on your phone that helps you find good companies and avoid the bad ones called Buycott. Look in your app store and you can download it for free. All you do is take a picture of the UPC and it will tell you if that company has GMO.
You can change your settings so that you know which companies voted against GMO labeling as well as containing them.