There is no easy way to measure how much the baby is getting, but this does not mean that you cannot know if the baby is getting enough. The same claim was made in 1900 and before. Modern formulas are only superficially similar to breastmilk. Every correction of a deficiency in formulas is advertised as an advance. Fundamentally they are inexact copies based on outdated and incomplete knowledge of what breastmilk is.
Formulas contain no antibodies, no living cells, no enzymes, no hormones.
They contain much more aluminum, manganese, cadmium and iron than breastmilk. They contain significantly more protein than breastmilk. The proteins and fats are fundamentally different from those in breastmilk. Your breastmilk is made as required to suit your baby. Formulas are made to suit every baby, and thus no baby.