Measuring %ABV of sweetened spirit

[azmotard]

I am tryong to find a precise way to measure the %ABV of Lemoncello that I am making.

I understand that an Alcholmeter or proof hydrometer can measure the %ABV of a spirit but only before flavors or sugars are added. Is there a way to accuratly meaure the %ABV once the spirit has been flavored and sweetend.

Some one mentioned that it could be done with a refractometer but I've not been able to find details on how to use one to do this measurement.