How are calories in food being measured?

The simplest way to know about calories in a particular food is by reading its label. If you want to know how labels are being prepared consider this:

One calorie is the amount of heat it takes to increase the temperature of 1 gram of water by 1 degree Celsius or 1.8 degrees Fahrenheit.

If you will burn a food to heat a certain amount of water and measure water temperature, you'll be able to assess how many calories this food contained.

But you don't have to because it's been done for you in labs. They know that 1 gram of carbohydrates has 4 calories, 1 gram of protein has 4 calories and 1 gram of fat has 9 calories, so the food labels are being created accordingly.