How is the alcohol content in wine determined?

Question

Some can be 3% and others 6%

Answer 1

Dunno if I've ever seen a 3% or a 6% wine.

Anyways, alcohol content is determined by the density. The density of the wine before it is fermented is taken and the density after it has fermented is taken. Because sugar water (essentially what the wine originally is) and wine have different densities the difference between initial and final density can be converted to a percentage.