Another way to figure this out (in case you are at the grocery store and don't have access to the tool) is to multiply the fat grams by 9 and multiply the protein by 4 (a fat gram has 9 calories and a gram of protein has 4). If these two totals added together equal the calories in a product than there are no hidden carbs. Example:

Calories: 74
Fat: 6
Protein:5
Do the math:
6X9= 54
5X4= 20
54 + 20 = 74

If you get a number higher than the total calories the product lists, you have hidden carbs. Divide the difference by 4 (because that's how many calories are in a gram of carbs) to get the additional "hidden" carbs you will be getting per serving.