I'm a Biologist and a Chemist. I have also questioned how much protein is in the plants I juice. I think the only way to be sure how much and which proteins and their building blocks amino acids are in a solution like juice is to submit it to a lab to test it. The US Pharmacopeia tells how to determine how much protein is in a solution. Then you could also digest the pulp left over and determine exactly what protein is there and which kinds. Because some protein is bound within cell membranes which would be excreted with the pulp. HPLC and MS would be the techniques that would determine the protein/amino acid content and what type of protein/amino acid it is.
From a health note I have found it better to use juice from fruits and veggies each day to replace one meal, and to eat my protein and veggies/fruits at another meal. This has made all of my health markers normal/almost normal range. Also it is important to make sure you consume foods with enough salt on a raw diet since it is a major electrolyte the body needs(salt=NaCl= Sodium atom + Chloride atom= sodium electrolyte and Chloride needed for HCl (hydrochloric acid synthesis for digestion in the stomach).