For one it depends on how accurate you need to be. If you are willing to assume that, for example, the overwhelming source of calories in soda is sugar, then you can just determine the amount of sugar and go from there.

For instance, if I wanted to determine the amount of calories in Coke, I would take a can of Coke, boil off the water in a tared container, find the residue mass, assume it is all sugar, convert to calories using the typical formula for number of calories per gram of sugar. This would give a decent estimate. If you wanted something more precise, you could boil off the contents, then determine the actual calorie count of the residue by calorimetry.

This doesn't quite work for alcoholic beverages because the alcohol boils off, but there are ways to determine alcohol content.