This unit of measure was named after Adolf F. Brix a 19th century scientist who developed the calibration in 1870 in order to determine the sugar content of liquids.
Measuring the Brix level of a plant is quick and relatively inexpensive. By using a refractometer and the plant sap/juice you can test for the degree of Brix, which is the sugar (or plant solids level) of the plant. Increased levels of Brix indicate a healthier plant and nutrient dense produce.
If you know how to identify good food, then you can buy it or grow it!
To measure, place a couple of drops of sap/juice to the prism of the refractometre and spread evenly. The refraction of the light altered by the sap/juice can be read from the scale you see when looking through the eye piece.