It depends how the soup is made. Do you make it yourself, or is it tinned?
In your country, are there nutrition labels on your packaged foods giving the ingredients and nutrition amounts? In the UK there are labels indicating how many carbs, fat, protein, fibre and other things are in the food.
Tinned lentils are about 16% carbs, but if the soup is made with flour for thickening, that also needs to be added to the carb content.