an inexpensive glass still and the required thermometer and hydrometer are what's needed. the procedure is in the gauging manual in 27cfr 30.32(c).
in the process, distillation is used to remove the sugars (that are more dense than water and will push your hydrometer up making the amount of alcohol seem lower) with water. then you measure the proof in the usual way.
let me say it again: replace the sugars with the amount of water the sugars displace.
the trouble is that while you are making the batch, you actually can do the weights and arithmetic quite accurately, and your results will be spot-on, but when gauging for tax, you are required to use the prescribed method noted above. you might as well spring for the glass still now.