1) Viscosity is one possibility. 2) Take samples, centrifuge and determine dry matter on the supernatant (using a balance or TGA). 3) Maybe try using Brix measured with a hand held refractometer. Never tried this, but it should work.

Centrifuge the mixture, then determine the dry weight of the precipitate (undissolved gum). Compare the weight of undissolved gum with the initial weight of the gum to determine the amount of gum dissolved in water.

I found the following paper where the authors used BCA test to determine GA concentration in water.

Article Biocompatible and bioactive gumArabic coated iron oxide magn...