When testing liqueurs, the sugar in the spirit affects the gravity and will likely be different to the specific gravity that you would find on an Alcometer.
The easiest way is to measure the percentage (%) of Alcohol by Volume (ABV) of the base spirit, is by using an alcometer (alcohol hydrometer), then calculate based on the volume of spirit compared to the volume of the finished drink.
Example: If you’re adding 645ml (8.3 US fl oz) of 40% ABV alcohol into 1.125L (38 US fl oz) of finished liqueur, the alcohol makes up approx. 57.3% of the drink, therefore 40% ABV x 57.3% = approx. 23 % ABV of the finished drink.