Notice the corn syrup in condiments: ketchup, salad dressing, peanut butter, jam, some pickles and mustards, etc. Notice the corn syrup in yogurts, ice creams, and frozen desserts. Notice the corn syrup in fruit drinks and flavored milk drinks. Notice the corn syrup in breakfast cereals and brand-name hearty-looking breads. Notice the corn syrup in brand-name cookies and crackers (Ritz, Wheat Thins, IIRC). Notice the corn syrup in canned pie fillings. Notice the corn syrup spaghetti sauces. Notice the corn syrup in canned soups.
Any food not located at a far outside aisle is likely to contain corn syrup. If a food has a brand name and it comes in a bottle, jar, tub, or box, it probably contains corn syrup. If a food comes in a can and it's not plain vegetables, it probably contains corn syrup.
I kind of exaggerate here, but not much. I notice that factory producers have been eliminating corn syrup from some foods in the past year or two. So, you might see "cane juice" or sugar in the ingredients list, instead of corn syrup. Cane juice is the new euphemism for sugar.
One of my Russian-American friends said about ten years ago, "I khate American food. Too sweet." It's "too sweet" because so many products contain sweeteners. If you're accustomed to American food, you probably don't notice the sweetness. Therefore, it wouldn't occur to you that it contains corn syrup (or sugar).
(HFCS is also, for some reason, a bit of a nutritional boogeyman for a lot of people. There's no actual evidence that consuming X calories of it is any worse for people than consuming X calories of sugar or honey, but it's the new "dioxin" in some circles.)
(I haven't actually seen this yet, but the thread reminded me of the trailer)
Here's a direct link to the trailer, since the above link is a bit slow: http://www.youtube.com/watch?v=UiCRwMMh9k8
Every packaged food in the US comes with an ingredient label.
More than likely up until lately you thought only hippies would be interested in knowing what's in their food.