In the U.S., producers of food that contains Genetically Modified Organisms (corn, soybeans, etc) are not required to label their food as genetically modified. Americans have been unknowingly consuming genetically modified foods for over a decade.
Now, however, produce that is genetically modified is sort-of labeled. It doesn't say "genetically modified" on it, but the PLU code on it will tell you if it is genetically modified. Most produce has a 4 digit PLU code. Organic produce has a 5 digit PLU code and the first number is a 9. Genetically engineered produce (tomatoes especially) have a 5 digit PLU code and the first number is an 8.
It's not much, but it's something.
Info on pg 17 of this document.