If you estimate that you have made about two liters of lemonade how do you check your estimate?

You would have to find some sort of graduated cylinder, or more probably a pitcher and pour your lemonade into it. Then you place your graduated cylinder on a level or at least fairly level surface and check the measurement of your lemonade with the top of the liquid at eye level. Or alternately pour it in a two liter bottle. Find a two liter bottle and pour the lemonade into it and see what happens.