@ Oscar
Refractometers are actually going mainstream, even for people without gardens.
they are being used to measure not only the quality of fruit, but also for the health of the plant, and this goes for non fruit plants also.
for example, herbs could be monitored, and by comparing brix, you pick herbs at their peak.
Simple refractometers, like those 40-50 dollar ones on ebay or amazon should do the trick. they are simple optical devices, all you do is squeeze a bit of liquid on the glass plate, which "bends" the light, the higher the brix, the more the light bends. accuracy I believe would depend more on the scale, I mean if you get one with a 0-50 scale, every little line mark may represent 2 brix. where if you get one close to the range we gardeners will be using which is about 0 - 18 brix range, you will be able to measure fractions of a 1 brix.