There are two ways to know whether a product in a grocery store is not GMO. First is that it is labeled "certified organic"; the second is that it is labeled "non-GMO project certified." As I blogged about in "The Cost of GMO Labeling" our food supply chain is a system of commingling of grain from farms throughout a region. Farmers grow several different varieties of, say, corn in a year, and unless they have a specialty contract with a specific buyer for that variety, all the corn gets delivered to the buyer's grain elevator—along with every other farmer in the region selling that same buyer’s corn. Farmers don't all grow the same varieties, but they all commingle in the grain-storage bin. Remember that grains, such as corn, are not fresh produce and, correctly stored, have a long storage time frame.
Tracing an ingredient like corn starch or soybean oil from an ingredient list on a processed product back to me, the farmer who supplied the raw product, would require a system of traceability wherein my grain was segregated by variety and by trait, from different varieties and then tracked step by step as it was harvested, transferred, shipped, unloaded, hulled, processed, and extruded; had its ingredients derived, blended, baked, fried, cooked, packaged, boxed, shipped and shelved; and then ultimately was purchased by a consumer. The presence of grain from my crop in the form of corn starch, for example, may be microscopic, and the DNA of any GMO crop may be so highly refined that it is no longer present. Thus, a system of labeling demands not just segregation but complete, truly accurate traceability in order for me to answer this question.