Thanksgiving is a time to enjoy the presence of family and reflect on another year of progress and health. It also allows us to eat more food in one sitting than is normally considered acceptable by unspoken societal rules. Of course, some people may think twice before eating just anything.

If GMO foods are on your list of foods to avoid -- whether you're wary of possible health risks, simply don't agree with agricultural methods created by Monsanto (NYSE:MON), Syngenta (NYSE: SYT), and Dow Chemical (NYSE:DOW); or would rather eat organic foods -- then you'll want to do your homework before making that last minute trip to the grocery store.

Is that even possible? Probably not. According to General Mills:

Because most U.S. farmers use GM seed to grow certain crops, if an American food or beverage product lists corn, soy, canola, cottonseed or beet sugar as an ingredient – and it is not organic – it likely contains GMOs. And 70 percent of foods on U.S. grocery store shelves likely do.

While I don't think biotech crops are anything to worry about, the fact remains that many consumers do. It will be difficult to completely avoid Monsanto during your Thanksgiving dinner, especially if you aren't the one buying the groceries. With that in mind, here's a list of the most likely GMO ingredients and the foods they're commonly found in that you can use as a rough guide while at the dinner table.

1. Soy

In 2012 about 94% of the soy produced in the United States was genetically modified, which ranked highest among all major crops. That makes it highly likely that any food product with soy ingredients that is not certified organic by the United States Department of Agriculture can be classified as a GMO food. Foods such as tofu and margarine are obvious candidates, but foods cooked with soybean-based vegetable oil or baked with soy flour also make the list.

2. Cottonseed

Approximately 90% of the domestic cotton crop in 2011 was genetically modified. Sure, it can be processed into a fine t-shirt, but it is also commonly used as cottonseed oil. It is widely used in margarine, salad dressings, and to fry potato chips. Remember that when snacking while watching your favorite NFL team.

3. Corn

In 2012 nearly 88% of the corn produced in the United States was genetically modified, which may be a surprise to many consumers who might otherwise think all of the nation's crop is engineered. It does, however, have the largest presence of the major crops in the food supply. You know the routine: most foods containing high fructose corn syrup and corn-flour (corn starch) can likely be classified as GMO foods. This will be the most difficult ingredient to avoid this Thanksgiving.

4. Canola oil

In 2010 nearly 90% of the domestic canola crop, which is primarily processed into canola oil for cooking, was genetically modified. Avoid foods cooked or fried in canola oil that are not certified organic to have the best chance of avoiding engineered canola.

5. Milk

Believe it or not, some cows are given recombinant bovine growth hormone to increase milk production. Numerous studies have yet to find health risks posed by consuming growth factors present in milk produced by cows injected with the hormone, but it still may not jive with your view of how food should be produced. Thus, you'll want to avoid drinking milk that isn't an organic brand, such as Horizon.

Foolish bottom line

Simply put, biotech crops are so widespread in American agriculture that it will be nearly impossible to completely avoid consuming GMO ingredients and GMO foods this Thanksgiving without buying strictly organic products. If you're attending a relative's for Thanksgiving dinner and have little or no control over foods being purchased, then you'll want to use the list above before asking your Uncle Joe to pass the salad dressing or margarine. Regardless of your stance on the issues at hand, enjoy a Happy Thanksgiving from myself and everyone at The Motley Fool!