The main thing that people on low-carb diets are concerned about is the impact of the carbs we eat on our blood sugar (blood glucose). We aim for blood sugar impact that is both low and slow. What we want to avoid are high peaks in blood glucose, and there is quite a bit of information on the label that can help us.
The grams of total carbohydrate is the first thing to look at. If it's very high, you can almost always put that food right back on the shelf. Even if the carbohydrates are coming from a "good" (nutritious) source, too much carbohydrate at once will shoot blood glucose up. The exact amount that is "too much" for you will depend on your own body's ability to tolerate glucose, and the particular diet you are on.
Beneath the Total Carbohydrates line in this section there will be two or three other lines - fiber, sugars, and sometimes sugar alcohols. You may notice that these figures do not add up to the total. This is because starch is not listed on food labels. Therefore, any missing carbohydrate can be assumed to be starch. In processed foods, starch (which is made up of long strands of glucose) generally raises blood glucose as much or more than sugars.
By now you may have figured out that the amount of sugar in a food is not a very reliable indicator of how much the food will affect blood sugar, because starches, and sometimes sugar alcohols, have similar effects.