Like how do we know an apple, for example, has so many calories, or a potato has so many grams of carbs. What does a person use to measure/ work out these values?
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Scientist use [Calorimetry](https://en.m.wikipedia.org/wiki/Calorimetry) to assess the caloric value of foods.
They do this by burning the food in a calorimeter, basically a test chamber very sensitive to heat, and then measure the heat given off by that food.
We know that a calorie is the energy required to heat 1 gram of water by 1C. (The kcal, or kilocalorie is the unit used for nutrition and is the same energy needed to heat 1 kilogram of water by 1C). So we can convert that heat energy into units of calories and get a nutritional value for that food.
Proteins, carbohydrates, and other values are assessed by laboratory methods that normally involve breaking down the food to its basic compounds and measuring their values analytically. Protein, for example, is usually determined by the [Kjeldahl Method](https://en.m.wikipedia.org/wiki/Kjeldahl_method), which involves digesting the food in acid, reacting it in stages, and then measuring nitrogen content to extrapolate for protein content.
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