https://haccpmentor.com/how-to-calculate-your-product-nutritional-information/
This would help you.
Basically you can either work backwards from the ingredients & processing losses and gains to estimate it using a nutrient software calculator.
Or you can send it to a nutritional lab, where they will test your product using the approved methods(chemical composition analysis of some sort.)
they use a calorimeter, which is basically a sealed space where food can be burned and the amount of energy put off (in heat) is used to determine the energetic (or caloric) value of it. there’s usually water above the burning food; one calorie raises 1g of water 1 degree C. the calories we use in food are actually Calories (the c is capitalized), and refers actually to a kilocalorie, or the energetic amount to raise 1kg of water 1 degree C
I don’t know about the rest of the world, but as someone has already said in detail, in Europe each of the individual macronutrients is analysed, and then a simple calculation is performed to find out the energy. For Food labelling purposes calorimeters aren’t used.
If you just want to calculate the energy without having to do a lab analysis, then using McCance and Widdowson’s “The composition of foods” is a acceptable alternative.
Calorimeter.
A calorie is the amount of energy required to raise 1 liter of water by 1 degree centigrade.
A sample of whatever is combusted in an enclosed chamber submerged in water. The temp change of the water is monitored.