As reported by Maria Gallagher, in the June 26, 2002 issue of the Philadelphia Inquirer, by reading the PLU code, you can tell if the fruit was genetically modified, organically grown or produced with chemical fertilizers, fungicides, or herbicides.
Here's what to look for. Look for the labels (stickers) stuck on your fruits and veggies:
The numeric system was developed by the Produce Electronic Identification Board, an affiliate of the Produce Marketing Association, a Newark, Delaware-based trade group for the produce industry. As of October 2001, the board had assigned more than 1,200 PLUs for individual produce items.
Genetically modified (GM) foods are food items that have had their DNA changed through genetic engineering. What this does is create food that is better suited to withstand environmental forces such as drought and bugs. In the US, by 2006 89% of the planted area of soybeans, 83% of cotton, and 61% maize were genetically modified varieties.
GM foods were first put on the market in the early 1990s. The first commercially grown genetically modified whole food crop was a tomato (called FlavrSavr), which was modified to ripen more slowly by Californian company Calgene. The most common modified foods are derived from plants: soybean, corn, canola, and cotton seed oil.
Here is how some of these foods become GM. Let's take soybeans for example, my father-in-law is a large scale farmer in Iowa. The corn and beans he purchases have been soaked in RoundUp. RoundUp is a commercial weed killer. When the weeds grow they spray the entire field with RoundUp and the crops are resistant to the weed killer, and only the weeds die. The farmers know this is a problem, but here's the catch, they can only purchase RoundUp ready seeds.
The issue with GM food lies with a problem called Gene Transfer. This happens when genetic material from the crop can be found in the human.
Currently there are only a few dozen peer reviewed studies completed on the health effects of genetically modified foods. The results of many of these studies strongly challenges the industry and government standard of substantial equivalence. As of January 2009 there has only been one human feeding study conducted on genetically modified foods. The study involved seven human volunteers who had their small intestines removed. These volunteers were to eat GM Soy to see if the DNA of the GM soy transferred to the human gut bacteria. Researchers identified that three of the seven volunteers had transgenes from GM soy transferred into their gut bacteria. "This transgene was stable inside the bacteria and appeared to produce herbicide-tolerant protein... In the only human feeding study ever conducted on GM crops, long standing assumptions that genes would not transfer to human gut bacteria were overturned.
It's a catch 22, clearly millions of people around the world would die if we didn't produce food on a large scale, but the side-effects are still largely unknown. I choose to avoid GM food as much as possible. The fact is if you live in America, the chances of you consuming GM is great.
 Adoption of Genetically Engineered Crops in the U.S. USDA ERS July 14, 2006
 Martineau, Belinda (2001). First Fruit: The Creation of the Flavr Savr Tomato and the Birth of Biotech Foods. McGraw-Hill. pp. 269. ISBN 978-0071360562.
 Netherwood et al., "Assessing the survival of transgenic planic plant DNA in the human gastrointestinal tract," Nature Biotechnology 22 (2004):2.