> Packaging on food also always has calories, so clearly people know how to estimate it fairly accurately.

That's not done by looking at it and guessing (or at least it _shouldn't_ be; manufacturers have been known to do this but it's bad practice and may cause them regulatory problems). In an ideal world it's done with one of these: https://en.wikipedia.org/wiki/Calorimeter ; less ideally it can be estimated based on the ingredients.