The Surgeon General is urging a warning label be placed on alcohol...

[ProfessorGAC]

22. Has To Be Under 1% Total

Additives have to combine to under 1%, otherwise the law requires them to be listed on the label.
Because of the spread of the analytical tests on the finished product, nobody would put in even 0.9%.
For tequila, the predominant additive is oak or mesquite extract to accentuate the flavors from the aging barrels. There's some in there anyway (ethanol is a pretty good extraction solvent), but to get the desired flavor profile, it would have to age more than what Mexican law requires. Extracting those flavors with steam from the same wood as the casks is quite fast & simple.
So yes, there are additives but typically they're alternate sourced natural products. (Sugar syrup is another example.)
Other rules limit ut in other liquors based on what's allowed, for instance, to call scotch, scotch. Or bourbon, bourbon. But, those rules shift pretty dramatically from one spirit type to another. (Scotch can't be called scotch unless the only thing added is caramel coloring.)
We learned a lot of GC/MS technique by running liquor samples. To train new people, we actually had Erlenmeyer flasks of various liquors in the lab. Looked better than liquor bottles at work!