1. Refined and processed carbohydrates, including both sugars and refined grains. "Sugar" can go by a lot of different names, (e.g. high fructose corn syrup), many of which are signs of a highly processed food. Here is a list of ingredients which essentially mean "sugar." Note that on this label we see "evaporated cane juice" - another name for sugar.
Processed starches often take the form of wheat or other flours. Processing grains or grinding them into flour makes them more glycemic. Note that the first ingredient in the sample label is "wheat flour." This almost always means "white flour," otherwise it would say "whole wheat flour". If the label doesn't say the grain is "whole" you can assume it isn't.
2. Sugar Alcohols - As noted previously, carefully check which sugar alcohol are in the ingredient list. This label has erythritol, which is a truly low-impact sugar alcohol, so that's a good one from the standpoint of low-carb eating.
3. Special Low-Carb Ingredients - There are some other special ingredients that are put into low-carb products to maintain taste or texture without raising blood sugar, such as artificial sweeteners. On this label we see inulin and wheat gluten. Wheat gluten is the protein part of the wheat. Inulin provides sweetness and texture. It's a good idea to get familiar with some of these special ingredients -- this article can help.
4. Partially hydrogenated oils are trans fats. Avoid any food with this ingredient.