Final product Brix?

[Ken]

.

[Charles@AEppelTreow]

Distill it quantitatively, return what's left in the pot to its original volume, cool and try your refractometer again. You can do this with about 250mL easily with benchtop lab equipment.

You might also try a colormetric assay - (glucose testing for diabetics, cheap and easy, but not real accurate and getting harder to find) or a spectrometer.

I've heard of small additions of sugar and/or citric acid to eaux de vie - but only very small ones - 0.5% or so.