While I don't like the idea of genetically modified seeds/plants I didn't think they could do much harm until I watched the movie Consumed (it's available on Netflix). Here's the basic rundown on GMOs. The only testing is done by the actual companies themselves (no government authorities). Seeds are modified to supposedly make them resistant to virus and pests and the companies are allowed to patent these seeds. The companies strictly enforce stopping the reuse of seeds which means that any farmers who inadvertently grow the seeds that wind or rain may have distributed from other farms, are sued for not paying for seeds that the organic farmers don't want on their land anyway. According to the movie, allergy related illnesses have doubled since the use of GMOs and many other illnesses have resulted. You can download a free book about making better choices for buying non-GMO foods here.
Note: The word "natural" on foods has no meaning. For instance one poultry manufacturer has both the word natural on their labels and insists that by federal law there are no antibiotics in the chickens that leave the manufacturing plant. What they don't tell you is that they do use antibiotics but, as the law states, are not used within a certain period of slaughtering in order to "flush" it from the meat. What you want to look for is "no antibiotics ever."
There is a list of non-GMO companies here. Support companies who voluntarily show their products do not contain genetically modified foods.
Many countries require labeling on packages showing if they contain genetically modified food.
The U.S. at present does not.