How Do You Know How Many Calories Are in a Serving?

As an early trainer, back in the day, we didn't have these aps that you can enter your food in. We had a this, there was a book, a thing twas alled calory king. And i'd have to go in there and look up the food or whatever. I think it's important to know, for example, how many caleries are in a ving of, you know, a common food that you may eat. The second thing i would say, depending on the person whom i'm talking to, half a gallon or a gallon of water, a day.