carbohydrate content = sugar content + starch content
That's the general idea.
carb calories = starch calories + sugar calories
carb weight = starch weight + sugar weight
but other times
carb weight = starch weight + sugar weight + fiber weight
In other words sometimes they DO count fiber in with starch and sugar and sometimes they DO NOT when calculating total carbs on the label, just to make things more confusing
However the fiber doesn't have noticeable calories, while both sugars and starches both have 4 calories per gram. So if you know the sugar and total carb calories
, then you know that any remaining carb calories come from starch, not fiber.
One thing to watch out for when calculating starch from labels is that the info is only approximate. Also there is a lot of "cheating" by picking very small serving sizes, because then they are allowed to round down anything under 1g to "zero".
Do you think cream has no carbs, because the label always says "0g carb"? Surprise!!!
Up to 4% of the total calories comes from carbs, but in the customary tiny serving size of 1 tablespoon they manage to round it down to zero. At least that's what they do in the US; I've heard from Alinus that European nutrition reporting tends to be more accurate.
The same is true for most cheeses, only a few really have close to zero carbs -- most have a small percentage of lactose remaining. Which may or may not be a problem for you; depends on how efficient your digestion is at metabolizing lactose sugar. Some of us need to avoid lactose about as diligently as starch (e.g. I make home-made yogurt with lactose-free milk because store-bought had too much lactose).
Anyways my point is that you can get starch estimates from labels, but it still takes some careful experiments to discover whether a particular food is really good for you. I take it as a first cut, to determine whether it is even worth trying something out.