How can I determine whether infused spirits contain sugar (before buying)? I'm sick of buying infused spirits (usually advertised in the U.S. as flavored vodkas) and discovering that they have added sugar (and often, apparently, glycerin).

I have scoured labels and asked in stores and never found any way of determining whether a liquor is a pure distillate or whether it has substantial additives (other than water and flavor).

In the U.S. is there any way to determine this before acquiring a particular bottle?
There is no way to determine if a particular flavored vodka has added sugar or glycerin! 

Unless one can find a particular website of a certain vodka, which states that there is no added sugar or glycerin the chances are that you will have to taste the product yourself in order to know.

The following information is from Drink Skool:


  If you’re confused, don’t be angry. Clearly the U.S. government is struggling with the issue as much as you are. Today’s science is capable of measuring imperfections in a spirit. So rather than insisting that 190 proof is an adequate minimum to guarantee quality in vodka, other metrics might have been considered in the TTB’s rule-making. For instance, some successful brands add sugar or glycerin to their vodkas to “smooth” them out. Never mind that these spirits throw any standard cocktail recipe completely out of balance. The larger question is: why do they need to add these softeners? What are they covering up? If they focused upon good raw ingredients and clean distillation they could achieve the same result. But perhaps that’s asking too much; it always comes back to economics.
  
  Other countries are demanding that producers note upon the label any additions such as sugar or glycerin; the TTB hasn’t wised up yet. It will probably be a while before those additives are visible on the vodka labels, but until then, taste the stuff yourself. If it tastes sweet, it probably is, and you may want to alter any cocktail recipes that you’re working from.


Absolut Raspberri is just one example of a flavoured vodka that does not contain added sugar!


  Absolut Raspberri is made exclusively from natural ingredients, and unlike some other flavored vodkas, it doesn’t contain any added sugar. It’s rich and intense with the fresh and fruity character of ripened raspberries.