Well you have to be tracking your food intake somehow and then it depends how pedantic you want to be. But the basic principle is that if you know the mass of each item of food you have consumed and you know the macronutrient content of each item of food you can aggregate your total daily consumption of carbs, protein, fat and calories. I do it this way because I find it interesting but it takes a fair amount of effort and I'm sure not everyone has the time or patience for this. But maybe there are some easier short-cut methods that some else can suggest.