I'm not sure I understand the question but I think it is something like "If I light some Sambucca, extinguish the flame after a couple of seconds and then measure the specific gravity of what's left can I estimate the remaining alcohol?". Practically speaking the answer is no in this case because all the sugar obscures the effects on the alcohol. If the question were the same except that the beverage were Vodka then the answer would be yes. As vodka can be considered a binary mixture of water and ethanol a density measurement would be considered adequate for 'proofing' before or after partial combustion of the alcohol.
Where the obscuration is high (Sambucca) TTB would make you separate the alcohol from the liquor before the density measurement.
Depending on how much accuracy you need that is doable. Obtain a Tralle hydrometer. This is one designed especially for proofing. The reason for the accuracy comment is that the density of aqueous solutions of alcohol are not much different from that of water until the proof becomes appreciable meaning that the marks for the lower percentages are very close together and hard to read.