Heirloom seeds can be found in organic produce. They were grown the natural, old-fashion way with soil, water, and sunlight. Open pollination is also important with these seeds. Naturally, these fruits and vegetables have the most richest flavors and resist pests and extreme weather. The only disadvantage is that they're not as disease resistant as hybrids.
Hybrids are created when pollination is artificially crossed between two plants. The color, shape, and sturdiness is stronger than heirloom plants. They're the most common produce in supermarkets. Although more disease-resistant, there's no variety in their flavor and the first generation of seeds cannot be replanted. So even if your fruit has seeds, it doesn't mean that you can plant them and have it flourish with fruit.
Genetic engineering has artificially altered these type of seeds. They're modified to have certain traits that the organic version would not have. Remember, just because it looks "natural" doesn't mean it is. There have been no known advantages to these fruits and vegetables, but plenty of problems. Not only have they not been approved for consumption by organic companies, they've been linked with infertility, accelerated aging, immune problems, changes in vital organs, and more. GMO's can also cross with non-GMO plants, so be careful with what you buy!
(source credit goes to Dr. J. Renae Norton. http://www.eatingdisorderpro.com/)
One way of checking what type of seeds your food has is by planting them. This will take some time, but if the fruit or vegetable grows back, then you automatically know that it's a much healthier choice. Two other results can be that the plant/ tree grows but doesn't sprout any fruits or vegetables, or nothing grows at all.
Have you ever tried planting the seeds from the fresh produce that you've bought? If so, what ended up happening? I'd really love to find out! My mom planted an avocado seed and a tree grew, but avocados take some time to sprout.